diff --git a/Doc/whatsnew/3.16.rst b/Doc/whatsnew/3.16.rst
index 09477b22d95413f..925f425aba58693 100644
--- a/Doc/whatsnew/3.16.rst
+++ b/Doc/whatsnew/3.16.rst
@@ -1295,11 +1295,6 @@ Build changes
 
   (Contributed by Stan Ulbrych, Florian Mayer and AnnaAr321 in :gh:`156049`.)
 
-* Update the bundled `mimalloc library`_ from version 2.1.2 to 2.5.2.
-  (Contributed by Donghee Na in :gh:`159111`.)
-
-  .. _mimalloc library: https://github.com/microsoft/mimalloc/
-
 
 C API changes
 =============
diff --git a/Include/internal/mimalloc/mimalloc/internal.h b/Include/internal/mimalloc/mimalloc/internal.h
index eb4b8c2d5dc1f02..83bf6348a5561c3 100644
--- a/Include/internal/mimalloc/mimalloc/internal.h
+++ b/Include/internal/mimalloc/mimalloc/internal.h
@@ -102,6 +102,8 @@ terms of the MIT license. A copy of the license can be found in the file
 #define mi_decl_externc
 #endif
 
+// CPython: upstream defines __wasi__ on Emscripten here. It is removed so that
+// CPython code can distinguish the two platforms (gh-158628).
 
 // --------------------------------------------------------------------------
 // Internal functions
diff --git a/Objects/mimalloc/os.c b/Objects/mimalloc/os.c
index 9c9683ce594f345..ab1813a74724417 100644
--- a/Objects/mimalloc/os.c
+++ b/Objects/mimalloc/os.c
@@ -276,6 +276,7 @@ static void* mi_os_prim_alloc_aligned(size_t size, size_t alignment, bool commit
   else {
     // if not aligned, free it, overallocate, and unmap around it
     #if !MI_TRACK_ASAN
+    // CPython: downgraded from _mi_warning_message, the warning is noisy in the test suite (gh-112027)
     _mi_verbose_message("unable to allocate aligned OS memory directly, fall back to over-allocation (size: 0x%zx bytes, address: %p, alignment: 0x%zx, commit: %d)\n", size, p, alignment, commit);
     #endif
     if (p != NULL) { mi_os_prim_free(p, size, (commit ? size : 0)); }
diff --git a/Objects/mimalloc/prim/unix/prim.c b/Objects/mimalloc/prim/unix/prim.c
index 87b3c96ea6045c8..c66198464e72a21 100644
--- a/Objects/mimalloc/prim/unix/prim.c
+++ b/Objects/mimalloc/prim/unix/prim.c
@@ -124,7 +124,7 @@ static bool unix_detect_overcommit(void) {
   #if defined(__linux__)
     int fd = mi_prim_open("/proc/sys/vm/overcommit_memory", O_RDONLY);
     if (fd >= 0) {
-      char buf[32] = {0};
+      char buf[32] = {0};  // CPython: zero-initialize to avoid an MSAN report
       ssize_t nread = mi_prim_read(fd, &buf, sizeof(buf));
       mi_prim_close(fd);
       // <https://www.kernel.org/doc/Documentation/vm/overcommit-accounting>
@@ -871,6 +871,7 @@ bool _mi_prim_random_buf(void* buf, size_t buf_len) {
 #elif defined(__ANDROID__) || defined(__DragonFly__) || \
       defined(__FreeBSD__) || defined(__NetBSD__) || defined(__OpenBSD__) || \
       defined(__sun) || \
+      defined(__CYGWIN__) /* CPython: gh-148461 */ || \
       (defined(__APPLE__) && (MAC_OS_X_VERSION_MIN_REQUIRED >= MAC_OS_X_VERSION_10_7))
 
 bool _mi_prim_random_buf(void* buf, size_t buf_len) {
diff --git a/Objects/mimalloc/random.c b/Objects/mimalloc/random.c
index b216c4c3505568c..f19520d930cb995 100644
--- a/Objects/mimalloc/random.c
+++ b/Objects/mimalloc/random.c
@@ -178,7 +178,7 @@ uintptr_t _mi_os_random_weak(uintptr_t extra_seed) {
 }
 
 static void mi_random_init_ex(mi_random_ctx_t* ctx, bool use_weak) {
-  uint8_t key[32] = {0};
+  uint8_t key[32] = {0};  // CPython: zero-initialize to avoid an MSAN report
   if (use_weak || !_mi_prim_random_buf(key, sizeof(key))) {
     // if we fail to get random data from the OS, we fall back to a
     // weak random source based on the current time