diff --git a/.github/workflows/ci-emscripten.yaml b/.github/workflows/ci-emscripten.yaml
new file mode 100644
index 00000000..96545571
--- /dev/null
+++ b/.github/workflows/ci-emscripten.yaml
@@ -0,0 +1,100 @@
+# Attributed to NumPy https://github.com/numpy/numpy/pull/25894
+# https://github.com/numpy/numpy/blob/d2d2c25fa81b47810f5cbd85ea6485eb3a3ffec3/.github/workflows/emscripten.yml
+#
+
+name: Pyodide CI
+
+on:
+  # TODO: refine after this is ready to merge
+  [push, pull_request, workflow_dispatch]
+
+env:
+  FORCE_COLOR: 3
+  # Disable instructions: AVX2 and SSE2 because Emscripten-specific SIMD
+  # support has not been implemented yet
+  DISABLE_NUMCODECS_AVX2: 1
+  DISABLE_NUMCODECS_SSE2: 1
+  # Common environment variables for both build and test jobs
+  PYODIDE_VERSION: 0.27.3
+  # PYTHON_VERSION and EMSCRIPTEN_VERSION are determined by PYODIDE_VERSION.
+  # The appropriate versions can be found in the Pyodide repodata.json
+  # "info" field, or in Makefile.envs:
+  # https://github.com/pyodide/pyodide/blob/main/Makefile.envs#L2
+  PYTHON_VERSION: 3.12 # any 3.12.x version works
+  EMSCRIPTEN_VERSION: 3.1.58
+  NODE_VERSION: 20
+
+concurrency:
+  group: ${{ github.workflow }}-${{ github.head_ref || github.run_id }}
+  cancel-in-progress: true
+
+permissions:
+  contents: read  # to fetch code (actions/checkout)
+
+jobs:
+  build-wasm-emscripten:
+    name: Build numcodecs Pyodide distribution
+    runs-on: ubuntu-22.04
+    # To enable this workflow on a fork, comment out:
+    # FIXME: uncomment after this is ready to merge
+    # if: github.repository == 'zarr-developers/numcodecs'
+    steps:
+      - name: Checkout source
+        uses: actions/checkout@v4
+        with:
+          submodules: recursive
+          fetch-depth: 0
+          # https://github.com/actions/checkout/issues/2041 isn't released yet
+          # fetch-tags: true # required for version resolution
+
+      - name: Set up Python ${{ env.PYTHON_VERSION }}
+        id: setup-python
+        uses: actions/setup-python@v5
+        with:
+          python-version: ${{ env.PYTHON_VERSION }}
+
+      - name: Set up Emscripten toolchain
+        uses: mymindstorm/setup-emsdk@v14
+        with:
+          version: ${{ env.EMSCRIPTEN_VERSION }}
+          actions-cache-folder: emsdk-cache
+
+      - name: Apply necessary patch(es)
+        run: |
+          patch -p1 < tools/ci/patches/0001-disable-multiprocessing-and-pthreads.patch
+          patch -p1 < tools/ci/patches/0002-add-missing-unistd-headers.patch -d c-blosc/internal-complibs/zlib-*/
+
+      - name: Install pyodide-build
+        run: python -m pip install pyodide-build
+
+      - name: Build numcodecs for Pyodide/WASM
+        run: pyodide build
+
+      - name: Set up Node.js
+        uses: actions/setup-node@v4
+        with:
+          node-version: ${{ env.NODE_VERSION }}
+
+      - name: Set up Pyodide virtual environment and test numcodecs for Pyodide
+        run: |
+          # Pin to a specific version of Pyodide to ensure reliability
+          pyodide xbuildenv install ${{ env.PYODIDE_VERSION }}
+
+          # Set up Pyodide virtual environment and activate it
+          pyodide venv .venv-pyodide
+          source .venv-pyodide/bin/activate
+
+          # For tests in test_zarr3.py
+          pip install zarr==3.0.0b1
+
+          # Install the built numcodecs WASM wheel and relevant dependencies
+          pip install $(ls dist/*.whl)"[msgpack,crc32c,pcodec,test,test_extras]"
+          # TODO: get zfpy built in Pyodide and install it here
+
+          # Change into a different directory before running tests to avoid
+          # the test runner picking up the local numcodecs package
+          cd docs
+
+          # Don't use the cache provider plugin, as it doesn't currently work
+          # with Pyodide: https://github.com/pypa/cibuildwheel/issues/1966
+          python -m pytest -p no:cacheprovider -svra --pyargs numcodecs
diff --git a/.github/workflows/ci.yaml b/.github/workflows/ci.yaml
index b1a1a7d9..cd2ae52d 100644
--- a/.github/workflows/ci.yaml
+++ b/.github/workflows/ci.yaml
@@ -25,7 +25,9 @@ jobs:
         uses: actions/checkout@v4
         with:
           submodules: recursive
-          fetch-depth: 0 # required for version resolution
+          fetch-depth: 0
+          # https://github.com/actions/checkout/issues/2041 isn't released yet
+          # fetch-tags: true # required for version resolution
 
       - name: Set up Conda
         uses: conda-incubator/setup-miniconda@v3.1.1
diff --git a/.gitignore b/.gitignore
index 6b7d99f1..80ce22b8 100644
--- a/.gitignore
+++ b/.gitignore
@@ -49,6 +49,7 @@ coverage.xml
 *,cover
 .hypothesis/
 cover/
+.pytest_cache/
 
 # Cython annotation files
 numcodecs/*.html
@@ -104,3 +105,6 @@ numcodecs/version.py
 
 # Cython generated
 numcodecs/*.c
+
+# Pyodide builds
+/.pyodide-xbuildenv-*
diff --git a/docs/contributing.rst b/docs/contributing.rst
index e5cd936e..d323926f 100644
--- a/docs/contributing.rst
+++ b/docs/contributing.rst
@@ -144,12 +144,11 @@ docstrings. The simplest way to run the unit tests is to invoke::
 
     $ pytest -v
 
-NumCodecs currently supports Python 6-3.9, so the above command must
+NumCodecs currently supports Python 3.8 and later, so the above command must
 succeed before code can be accepted into the main code base.
 
-All tests are automatically run via Travis (Linux) and AppVeyor (Windows) continuous
-integration services for every pull request. Tests must pass under both services before
-code can be accepted.
+All tests are automatically run via continuous integration services for every pull request
+provided by GitHub Actions. Tests must pass under both services before code can be accepted.
 
 Code standards
 ~~~~~~~~~~~~~~
@@ -163,11 +162,11 @@ Conformance can be checked by running::
 Test coverage
 ~~~~~~~~~~~~~
 
-NumCodecs maintains 100% test coverage under the latest Python stable release (currently
-Python 3.9). Both unit tests and docstring doctests are included when computing
-coverage. Running ``pytest -v`` will automatically run the test suite with coverage
-and produce a coverage report. This should be 100% before code can be accepted into the
-main code base.
+NumCodecs maintains 100% test coverage under the latest Python stable release.
+Both unit tests and docstring doctests are included when computing coverage. Running
+``pytest -v`` will automatically run the test suite with coverage and produce a
+coverage report. This should be 100% before code can be accepted into the main
+code base.
 
 When submitting a pull request, coverage will also be collected across all supported
 Python versions via the Codecov service, and will be reported back within the pull
@@ -179,7 +178,7 @@ Documentation
 Docstrings for user-facing classes and functions should follow the `numpydoc
 <https://numpydoc.readthedocs.io/en/latest/format.html#docstring-standard>`_ standard,
 including sections for Parameters and Examples. All examples will be run as doctests
-under Python 3.9.
+under a stable version of Python.
 
 NumCodecs uses Sphinx for documentation, hosted on readthedocs.org. Documentation is
 written in the RestructuredText markup language (.rst files) in the ``docs`` folder.
@@ -207,8 +206,8 @@ Pull requests submitted by an external contributor should be reviewed and approv
 one core developers before being merged. Ideally, pull requests submitted by a core developer
 should be reviewed and approved by at least one other core developers before being merged.
 
-Pull requests should not be merged until all CI checks have passed (Travis, AppVeyor,
-Codecov) against code that has had the latest main merged in.
+Pull requests should not be merged until all CI checks have passed (GitHub Actions,
+CodeCov) against code that has had the latest main merged in.
 
 Compatibility and versioning policies
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
diff --git a/docs/release.rst b/docs/release.rst
index 14fc9423..afecaf3f 100644
--- a/docs/release.rst
+++ b/docs/release.rst
@@ -20,8 +20,12 @@ Unreleased
 Enhancements
 ~~~~~~~~~~~~
 
+* Add official support for Emscripten (Pyodide) builds. These builds have
+  existed upstream/in-tree in Pyodide, and are now tested here more extensively.
+  By :user:`Agriya Khetarpal <agriyakhetarpal>`.
+
 * Add support for the Linux AArch64 architecture, and bump the minimum
-macOS deployment target for x86_64 to 10.13.
+  macOS deployment target for x86_64 to 10.13.
   By :user:`Agriya Khetarpal <agriyakhetarpal>`, :issue:`288`.
 
 Improvements
diff --git a/numcodecs/blosc.pyx b/numcodecs/blosc.pyx
index 3caa3607..04806b25 100644
--- a/numcodecs/blosc.pyx
+++ b/numcodecs/blosc.pyx
@@ -88,6 +88,8 @@ def get_mutex():
             mutex = None
         except ImportError:
             mutex = None
+        except ModuleNotFoundError:
+            mutex = None
         _MUTEX = mutex
         _MUTEX_IS_INIT = True
     return _MUTEX
diff --git a/numcodecs/tests/common.py b/numcodecs/tests/common.py
index bb7c4780..857f61c9 100644
--- a/numcodecs/tests/common.py
+++ b/numcodecs/tests/common.py
@@ -1,6 +1,8 @@
 import array
 import json as _json
 import os
+import platform
+import sys
 from glob import glob
 
 import numpy as np
@@ -26,6 +28,8 @@
     'เฮลโลเวิลด์',
 ]
 
+is_wasm = (sys.platform == 'emscripten') or (platform.machine() in ['wasm32', 'wasm64'])
+
 
 def compare_arrays(arr, res, precision=None):
     # ensure numpy array with matching dtype
diff --git a/numcodecs/tests/test_blosc.py b/numcodecs/tests/test_blosc.py
index 0bc14010..d546d0cb 100644
--- a/numcodecs/tests/test_blosc.py
+++ b/numcodecs/tests/test_blosc.py
@@ -19,6 +19,7 @@
     check_err_decode_object_buffer,
     check_err_encode_object_buffer,
     check_max_buffer_size,
+    is_wasm,
 )
 
 codecs = [
@@ -223,6 +224,7 @@ def _decode_worker(enc):
     return compressor.decode(enc)
 
 
+@pytest.mark.skipif(is_wasm, reason="WASM/Pyodide does not support multiprocessing")
 @pytest.mark.parametrize('pool', [Pool, ThreadPool])
 def test_multiprocessing(use_threads, pool):
     data = np.arange(1000000)
diff --git a/numcodecs/tests/test_entrypoints_backport.py b/numcodecs/tests/test_entrypoints_backport.py
index 7e1c32bc..ded5d33a 100644
--- a/numcodecs/tests/test_entrypoints_backport.py
+++ b/numcodecs/tests/test_entrypoints_backport.py
@@ -6,6 +6,7 @@
 import pytest
 
 import numcodecs.registry
+from numcodecs.tests.common import is_wasm
 
 importlib_spec = importlib.util.find_spec("importlib_metadata")
 if importlib_spec is None or importlib_spec.loader is None:  # pragma: no cover
@@ -29,6 +30,7 @@ def get_entrypoints_with_importlib_metadata_loaded():
     assert cls.codec_id == "test"
 
 
+@pytest.mark.skipif(is_wasm, reason="Spawning processes is not supported in Pyodide/WASM")
 def test_entrypoint_codec_with_importlib_metadata():
     p = Process(target=get_entrypoints_with_importlib_metadata_loaded)
     p.start()
diff --git a/numcodecs/tests/test_shuffle.py b/numcodecs/tests/test_shuffle.py
index 6e6d744a..dc2387b2 100644
--- a/numcodecs/tests/test_shuffle.py
+++ b/numcodecs/tests/test_shuffle.py
@@ -14,6 +14,7 @@
     check_backwards_compatibility,
     check_config,
     check_encode_decode,
+    is_wasm,
 )
 
 codecs = [
@@ -87,6 +88,7 @@ def _decode_worker(enc):
     return compressor.decode(enc)
 
 
+@pytest.mark.skipif(is_wasm, reason="WASM/Pyodide does not support multiprocessing")
 @pytest.mark.parametrize('pool', [Pool, ThreadPool])
 def test_multiprocessing(pool):
     data = np.arange(1000000)
diff --git a/numcodecs/tests/test_zarr3.py b/numcodecs/tests/test_zarr3.py
index 0d8ecc74..508f8563 100644
--- a/numcodecs/tests/test_zarr3.py
+++ b/numcodecs/tests/test_zarr3.py
@@ -5,6 +5,8 @@
 import numpy as np
 import pytest
 
+from numcodecs.tests.common import is_wasm
+
 if TYPE_CHECKING:  # pragma: no cover
     import zarr
 else:
@@ -53,6 +55,7 @@ def test_docstring(codec_class: type[numcodecs.zarr3._NumcodecsCodec]):
     assert "See :class:`numcodecs." in codec_class.__doc__  # type: ignore[operator]
 
 
+@pytest.mark.skipif(is_wasm, reason="Threads are not supported in Pyodide/WASM")
 @pytest.mark.parametrize(
     "codec_class",
     [
@@ -85,6 +88,8 @@ def test_generic_compressor(
     np.testing.assert_array_equal(data, a[:, :])
 
 
+# TODO: undo skips here when we can test async code in WASM
+@pytest.mark.skipif(is_wasm, reason="testing async code not yet supported in Pyodide/WASM")
 @pytest.mark.parametrize(
     ("codec_class", "codec_config"),
     [
@@ -124,6 +129,8 @@ def test_generic_filter(
     np.testing.assert_array_equal(data, a[:, :])
 
 
+# TODO: undo skips here when we can test async code in WASM
+@pytest.mark.skipif(is_wasm, reason="testing async code not yet supported in Pyodide/WASM")
 def test_generic_filter_bitround(store: StorePath):
     data = np.linspace(0, 1, 256, dtype="float32").reshape((16, 16))
 
@@ -142,6 +149,8 @@ def test_generic_filter_bitround(store: StorePath):
     assert np.allclose(data, a[:, :], atol=0.1)
 
 
+# TODO: undo skips here when we can test async code in WASM
+@pytest.mark.skipif(is_wasm, reason="testing async code not yet supported in Pyodide/WASM")
 def test_generic_filter_quantize(store: StorePath):
     data = np.linspace(0, 10, 256, dtype="float32").reshape((16, 16))
 
@@ -160,6 +169,8 @@ def test_generic_filter_quantize(store: StorePath):
     assert np.allclose(data, a[:, :], atol=0.001)
 
 
+# TODO: undo skips here when we can test async code in WASM
+@pytest.mark.skipif(is_wasm, reason="testing async code not yet supported in Pyodide/WASM")
 def test_generic_filter_packbits(store: StorePath):
     data = np.zeros((16, 16), dtype="bool")
     data[0:4, :] = True
@@ -189,6 +200,8 @@ def test_generic_filter_packbits(store: StorePath):
         )
 
 
+# TODO: undo skips here when we can test async code in WASM
+@pytest.mark.skipif(is_wasm, reason="testing async code not yet supported in Pyodide/WASM")
 @pytest.mark.parametrize(
     "codec_class",
     [
diff --git a/numcodecs/zarr3.py b/numcodecs/zarr3.py
index f1743ffb..e4c10ae4 100644
--- a/numcodecs/zarr3.py
+++ b/numcodecs/zarr3.py
@@ -25,6 +25,20 @@
 
 from __future__ import annotations
 
+# Short workaround for skipping the doctest above in a WASM environment
+# compiled via Emscripten where threads are not available, and accessing
+# the pytest config has no effect and ignoring warnings does not work.
+try:
+    import pytest
+
+    from numcodecs.tests.common import is_wasm
+
+    if is_wasm:  # pragma: no cover
+        pytest.skip("zarr3 doctests not supported in WASM", allow_module_level=True)
+# not running tests
+except (ImportError, ModuleNotFoundError):  # pragma: no cover
+    pass
+
 import asyncio
 import math
 from dataclasses import dataclass, replace
diff --git a/setup.py b/setup.py
index c27fad35..4efaf6f0 100644
--- a/setup.py
+++ b/setup.py
@@ -64,7 +64,8 @@ def blosc_extension():
     define_macros = []
 
     # ensure pthread is properly linked on POSIX systems
-    if os.name == 'posix':
+    # and not when compiling to WASM.
+    if os.name == 'posix' and "PYODIDE" not in os.environ:
         extra_compile_args.append('-pthread')
         extra_link_args.append('-pthread')
 
diff --git a/tools/ci/patches/0001-disable-multiprocessing-and-pthreads.patch b/tools/ci/patches/0001-disable-multiprocessing-and-pthreads.patch
new file mode 100644
index 00000000..dcab2de0
--- /dev/null
+++ b/tools/ci/patches/0001-disable-multiprocessing-and-pthreads.patch
@@ -0,0 +1,34 @@
+This patch disables multiprocessing and pthread for blosc. This file
+is adapted from and attributed to the Pyodide developers and can be
+viewed at the upstream Pyodide repository at the following link:
+
+https://github.com/pyodide/pyodide/blob/d32e376013d8977b66c6aa828042b1fee8047aea/packages/numcodecs/patches/fixblosc.patch
+
+
+diff --git a/c-blosc/blosc/blosc.h b/c-blosc/blosc/blosc.h
+index 40857d0..8a1e969 100644
+--- a/c-blosc/blosc/blosc.h
++++ b/c-blosc/blosc/blosc.h
+@@ -50,7 +50,7 @@ extern "C" {
+   ((INT_MAX - BLOSC_MAX_TYPESIZE * sizeof(int32_t)) / 3)
+
+ /* The maximum number of threads (for some static arrays) */
+-#define BLOSC_MAX_THREADS 256
++#define BLOSC_MAX_THREADS 1
+
+ /* Codes for shuffling (see blosc_compress) */
+ #define BLOSC_NOSHUFFLE   0  /* no shuffle */
+
+ diff --git a/c-blosc/blosc/blosc.c b/c-blosc/blosc/blosc.c
+index a5a5bd5..2a7797c 100644
+--- a/c-blosc/blosc/blosc.c
++++ b/c-blosc/blosc/blosc.c
+@@ -2236,6 +2236,7 @@ void blosc_atfork_child(void) {
+
+ void blosc_init(void)
+ {
++  g_initlib = 1;
+   /* Return if we are already initialized */
+   if (g_initlib) return;
+
+
diff --git a/tools/ci/patches/0002-add-missing-unistd-headers.patch b/tools/ci/patches/0002-add-missing-unistd-headers.patch
new file mode 100644
index 00000000..7bbc88c1
--- /dev/null
+++ b/tools/ci/patches/0002-add-missing-unistd-headers.patch
@@ -0,0 +1,51 @@
+This patch adds missing headers in vendored zlib as done in numcodecs.js. This file
+is attributed to the Pyodide developers and can be viewed at the upstream
+Pyodide repository at the following link;
+
+https://github.com/pyodide/pyodide/blob/d32e376013d8977b66c6aa828042b1fee8047aea/packages/numcodecs/patches/fixzlib.patch
+
+This patch is applied in the c-blosc/internal-complibs/zlib-<ZLIB_VERSION>/ directory
+in the .github/workflows/ci-emscripten.yaml workflow file.
+
+diff --git a/gzlib.c b/gzlib.c
+index fae202e..80606a6 100644
+--- a/gzlib.c
++++ b/gzlib.c
+@@ -2,7 +2,7 @@
+  * Copyright (C) 2004, 2010, 2011, 2012, 2013 Mark Adler
+  * For conditions of distribution and use, see copyright notice in zlib.h
+  */
+-
++#include <unistd.h>
+ #include "gzguts.h"
+
+ #if defined(_WIN32) && !defined(__BORLANDC__)
+
+diff --git a/gzread.c b/gzread.c
+index bf4538e..afe6acd 100644
+--- a/gzread.c
++++ b/gzread.c
+@@ -2,7 +2,7 @@
+  * Copyright (C) 2004, 2005, 2010, 2011, 2012, 2013 Mark Adler
+  * For conditions of distribution and use, see copyright notice in zlib.h
+  */
+-
++#include <unistd.h>
+ #include "gzguts.h"
+
+ /* Local functions */
+
+
+diff --git a/gzwrite.c b/gzwrite.c
+index aa767fb..a87676f 100644
+--- a/gzwrite.c
++++ b/gzwrite.c
+@@ -2,7 +2,7 @@
+  * Copyright (C) 2004, 2005, 2010, 2011, 2012, 2013 Mark Adler
+  * For conditions of distribution and use, see copyright notice in zlib.h
+  */
+-
++#include <unistd.h>
+ #include "gzguts.h"
+
+ /* Local functions */