diff --git a/.github/workflows/ci.yaml b/.github/workflows/ci.yaml
index 53f41dc85..b5bb507cc 100644
--- a/.github/workflows/ci.yaml
+++ b/.github/workflows/ci.yaml
@@ -12,13 +12,13 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        python-version: ["3.12", "3.13", "3.14"]
+        python-version: ["3.12", "3.13", "3.14", "3.14t"]
         # macos-15-large is an intel runner, macos-14 is an arm64 runner
         platform: [ubuntu-latest, ubuntu-22.04-arm, windows-latest, macos-15-large, macos-14]
 
     defaults:
       run:
-        shell: bash -el {0}
+        shell: bash -e {0}
 
     steps:
       - name: Checkout source
@@ -34,13 +34,18 @@ jobs:
           cache: 'pip'
 
       - name: Install numcodecs
+        if: ${{ !endsWith(matrix.python-version, 't') }}
         run: python -m pip install -v ".[test,test_extras,msgpack,google_crc32c,crc32c,pcodec,zfpy]"
 
+      - name: Install numcodecs (free-threaded)
+        # pcodec, zfpy and google_crc32c do not ship free-threaded wheels yet
+        if: ${{ endsWith(matrix.python-version, 't') }}
+        run: python -m pip install -v ".[test,test_extras,msgpack,crc32c]"
+
       - name: List installed packages
         run: python -m pip list
 
       - name: Run tests
-        shell: "bash -l {0}"
         run: pytest -v
 
       - uses: codecov/codecov-action@v6
diff --git a/.github/workflows/wheel.yaml b/.github/workflows/wheel.yaml
index 947650e40..d8fa6ab11 100644
--- a/.github/workflows/wheel.yaml
+++ b/.github/workflows/wheel.yaml
@@ -22,8 +22,10 @@ jobs:
         # macos-15-large is an intel runner, macos-14 is an arm64 runner
         os: [ubuntu-latest, ubuntu-22.04-arm, windows-latest, macos-15-large, macos-14]
     env:
-      CIBW_TEST_COMMAND: python -c "import numcodecs"
-      CIBW_BUILD: "cp312-* cp313-* cp314-*"
+      # on free-threaded builds also check that no extension re-enables the GIL
+      CIBW_TEST_COMMAND: python -c "import sys, sysconfig, numcodecs; assert not (sysconfig.get_config_var('Py_GIL_DISABLED') and sys._is_gil_enabled())"
+      CIBW_BUILD: "cp312-* cp313-* cp314-* cp314t-*"
+      CIBW_ENABLE: cpython-freethreading
       CIBW_SKIP: "*-musllinux_* *win32 *_i686 *_s390x"
       # note: cibuildwheel config-settings are set in pyproject.toml
 
diff --git a/pyproject.toml b/pyproject.toml
index ce472c983..185ecc3a5 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -3,7 +3,7 @@ requires = [
     "meson-python>=0.17",
     "meson>=1.6.0",
     "setuptools-scm>=6.2",
-    "Cython>=3.0",
+    "Cython>=3.1",
     "numpy>=2",
 ]
 build-backend = "mesonpy"
@@ -84,7 +84,7 @@ dev = [
     "meson-python>=0.17",
     "meson>=1.6.0",
     "ninja",
-    "cython>=3.0",
+    "cython>=3.1",
     "setuptools-scm>=6.2",
     "numpy>=2",
     "pytest==9.0.3",
diff --git a/src/numcodecs/_shuffle.pyx b/src/numcodecs/_shuffle.pyx
index 0f0dafeb8..667beb5ab 100644
--- a/src/numcodecs/_shuffle.pyx
+++ b/src/numcodecs/_shuffle.pyx
@@ -3,6 +3,7 @@
 # cython: linetrace=False
 # cython: binding=False
 # cython: language_level=3
+# cython: freethreading_compatible=True
 
 cimport cython
 
diff --git a/src/numcodecs/blosc.pyx b/src/numcodecs/blosc.pyx
index d61f73588..8c52dd95c 100644
--- a/src/numcodecs/blosc.pyx
+++ b/src/numcodecs/blosc.pyx
@@ -3,6 +3,7 @@
 # cython: linetrace=False
 # cython: binding=False
 # cython: language_level=3
+# cython: freethreading_compatible=True
 import threading
 import multiprocessing
 import os
@@ -72,22 +73,20 @@ AUTOSHUFFLE = -1
 # automatic block size - let blosc decide
 AUTOBLOCKS = 0
 
-# synchronization
-_MUTEX = None
-_MUTEX_IS_INIT = False
-
-def get_mutex():
-    global _MUTEX_IS_INIT, _MUTEX
-    if not _MUTEX_IS_INIT:
-        try:
-            mutex = multiprocessing.Lock()
-        except OSError:
-            mutex = None
-        except ImportError:
-            mutex = None
-        _MUTEX = mutex
-        _MUTEX_IS_INIT = True
-    return _MUTEX
+# computed once at import; blosc_list_compressors() fills a static buffer without locking
+_COMPRESSORS = tuple(blosc_list_compressors().decode('ascii').split(','))
+
+# serializes access to blosc's global context
+_MUTEX = threading.Lock()
+
+
+def _reset_mutex_after_fork():
+    global _MUTEX
+    _MUTEX = threading.Lock()
+
+
+if hasattr(os, 'register_at_fork'):
+    os.register_at_fork(after_in_child=_reset_mutex_after_fork)
 
 # store ID of process that first loads the module, so we can detect a fork later
 _importer_pid = os.getpid()
@@ -95,31 +94,34 @@ _importer_pid = os.getpid()
 
 def _init():
     """Initialize the Blosc library environment."""
-    blosc_init()
+    with _MUTEX:
+        blosc_init()
 
 
 def _destroy():
     """Destroy the Blosc library environment."""
-    blosc_destroy()
+    with _MUTEX:
+        blosc_destroy()
 
 
 def list_compressors():
     """Get a list of compressors supported in the current build."""
-    s = blosc_list_compressors()
-    s = s.decode('ascii')
-    return s.split(',')
+    return list(_COMPRESSORS)
 
 
 def get_nthreads():
     """Get the number of threads that Blosc uses internally for compression and
     decompression."""
-    return blosc_get_nthreads()
+    with _MUTEX:
+        return blosc_get_nthreads()
 
 
 def set_nthreads(int nthreads):
     """Set the number of threads that Blosc uses internally for compression and
     decompression."""
-    return blosc_set_nthreads(nthreads)
+    # blosc_set_nthreads re-creates the global context
+    with _MUTEX:
+        return blosc_set_nthreads(nthreads)
 
 
 def _cbuffer_sizes(source):
@@ -247,7 +249,7 @@ def compress(source, char* cname, int clevel, int shuffle=SHUFFLE,
 
     # check valid cname early
     cname_str = cname.decode('ascii')
-    if cname_str not in list_compressors():
+    if cname_str not in _COMPRESSORS:
         _err_bad_cname(cname_str)
 
     # obtain source memoryview
@@ -288,8 +290,8 @@ def compress(source, char* cname, int clevel, int shuffle=SHUFFLE,
 
             # N.B., we are using blosc's global context, and so we need to use a lock
             # to ensure no-one else can modify the global context while we're setting it
-            # up and using it.
-            with get_mutex():
+            # up and using it (including set_nthreads/_destroy re-initializing it).
+            with _MUTEX:
 
                 # set compressor
                 compressor_set = blosc_set_compressor(cname)
@@ -385,8 +387,9 @@ def decompress(source, dest=None):
         # perform decompression
         if _get_use_threads():
             # allow blosc to use threads internally
-            with nogil:
-                ret = blosc_decompress(source_ptr, dest_ptr, nbytes)
+            with _MUTEX:
+                with nogil:
+                    ret = blosc_decompress(source_ptr, dest_ptr, nbytes)
         else:
             with nogil:
                 ret = blosc_decompress_ctx(source_ptr, dest_ptr, nbytes, 1)
@@ -410,10 +413,6 @@ def _get_use_threads():
     global use_threads
     proc = multiprocessing.current_process()
 
-    # check if locks are available, and if not no threads
-    if not get_mutex():
-        return False
-
     # check for fork
     if proc.pid != _importer_pid:
         # If this module has been imported in the parent process, and the current process
diff --git a/src/numcodecs/compat_ext.pyx b/src/numcodecs/compat_ext.pyx
index 05dc2bf42..9671afde0 100644
--- a/src/numcodecs/compat_ext.pyx
+++ b/src/numcodecs/compat_ext.pyx
@@ -3,6 +3,7 @@
 # cython: linetrace=False
 # cython: binding=False
 # cython: language_level=3
+# cython: freethreading_compatible=True
 
 from cpython.buffer cimport PyBuffer_IsContiguous
 from cpython.memoryview cimport PyMemoryView_GET_BUFFER
diff --git a/src/numcodecs/fletcher32.pyx b/src/numcodecs/fletcher32.pyx
index 56a5f793c..1ecbb45d6 100644
--- a/src/numcodecs/fletcher32.pyx
+++ b/src/numcodecs/fletcher32.pyx
@@ -1,6 +1,7 @@
 # cython: language_level=3
 # cython: overflowcheck=False
 # cython: cdivision=True
+# cython: freethreading_compatible=True
 
 
 from libc.stdint cimport uint8_t, uint16_t, uint32_t
diff --git a/src/numcodecs/jenkins.pyx b/src/numcodecs/jenkins.pyx
index 0efaf26ce..acdaa1c1b 100644
--- a/src/numcodecs/jenkins.pyx
+++ b/src/numcodecs/jenkins.pyx
@@ -1,6 +1,7 @@
 # cython: language_level=3
 # cython: overflowcheck=False
 # cython: cdivision=True
+# cython: freethreading_compatible=True
 
 """
 Cython implementation of Bob Jenkin's hashlittle from lookup3.c.
diff --git a/src/numcodecs/lz4.pyx b/src/numcodecs/lz4.pyx
index 9b1b431c8..6eac246b5 100644
--- a/src/numcodecs/lz4.pyx
+++ b/src/numcodecs/lz4.pyx
@@ -3,6 +3,7 @@
 # cython: linetrace=False
 # cython: binding=False
 # cython: language_level=3
+# cython: freethreading_compatible=True
 
 
 from libc.stdint cimport uint8_t, uint32_t
diff --git a/src/numcodecs/vlen.pyx b/src/numcodecs/vlen.pyx
index 69ae620a8..f51e8cad7 100644
--- a/src/numcodecs/vlen.pyx
+++ b/src/numcodecs/vlen.pyx
@@ -3,6 +3,7 @@
 # cython: linetrace=False
 # cython: binding=False
 # cython: language_level=3
+# cython: freethreading_compatible=True
 
 
 cimport cython
diff --git a/src/numcodecs/zstd.pyx b/src/numcodecs/zstd.pyx
index 7e070a62a..7b6ff3c02 100644
--- a/src/numcodecs/zstd.pyx
+++ b/src/numcodecs/zstd.pyx
@@ -3,6 +3,7 @@
 # cython: linetrace=False
 # cython: binding=False
 # cython: language_level=3
+# cython: freethreading_compatible=True
 
 
 from cpython.bytes cimport PyBytes_AS_STRING, PyBytes_FromStringAndSize