diff --git a/Doc/library/multiprocessing.shared_memory.rst b/Doc/library/multiprocessing.shared_memory.rst
index e8f04a6ac7b95d..533b2e00253c53 100644
--- a/Doc/library/multiprocessing.shared_memory.rst
+++ b/Doc/library/multiprocessing.shared_memory.rst
@@ -311,28 +311,34 @@ finishes execution.
    existing :class:`!ShareableList`, specify its shared memory block's unique
    name while leaving *sequence* set to ``None``.
 
+   .. versionchanged:: 3.15
+      Trailing null bytes (``\x00``) in :class:`bytes` and :class:`str` values
+      are now preserved correctly. See :gh:`106939` and :gh:`145261`.
+
    .. note::
 
-      A known issue exists for :class:`bytes` and :class:`str` values.
-      If they end with ``\x00`` nul bytes or characters, those may be
-      *silently stripped* when fetching them by index from the
-      :class:`!ShareableList`. This ``.rstrip(b'\x00')`` behavior is
-      considered a bug and may go away in the future. See :gh:`106939`.
+      In Python 3.14 and earlier, a known issue exists for :class:`bytes` and
+      :class:`str` values. If they end with ``\x00`` nul bytes or characters,
+      those may be *silently stripped* when fetching them by index from the
+      :class:`!ShareableList`. This ``.rstrip(b'\x00')`` behavior was fixed
+      in Python 3.15.
 
-   For applications where rstripping of trailing nulls is a problem,
-   work around it by always unconditionally appending an extra non-0
-   byte to the end of such values when storing and unconditionally
-   removing it when fetching:
+   For applications that need to work with Python 3.14 and earlier where
+   rstripping of trailing nulls is a problem, work around it by always
+   unconditionally appending an extra non-0 byte to the end of such values
+   when storing and unconditionally removing it when fetching:
 
    .. doctest::
 
        >>> from multiprocessing import shared_memory
-       >>> nul_bug_demo = shared_memory.ShareableList(['?\x00', b'\x03\x02\x01\x00\x00\x00'])
-       >>> nul_bug_demo[0]
-       '?'
-       >>> nul_bug_demo[1]
-       b'\x03\x02\x01'
-       >>> nul_bug_demo.shm.unlink()
+       >>> # Python 3.15+: trailing nulls are preserved
+       >>> sl = shared_memory.ShareableList(['?\x00', b'\x03\x02\x01\x00\x00\x00'])
+       >>> sl[0]
+       '?\x00'
+       >>> sl[1]
+       b'\x03\x02\x01\x00\x00\x00'
+       >>> sl.shm.unlink()
+       >>> # Workaround for Python 3.14 and earlier:
        >>> padded = shared_memory.ShareableList(['?\x00\x07', b'\x03\x02\x01\x00\x00\x00\x07'])
        >>> padded[0][:-1]
        '?\x00'
diff --git a/Lib/multiprocessing/shared_memory.py b/Lib/multiprocessing/shared_memory.py
index 99a8ce3320ad4e..80fe66a1f86cb9 100644
--- a/Lib/multiprocessing/shared_memory.py
+++ b/Lib/multiprocessing/shared_memory.py
@@ -286,8 +286,8 @@ class ShareableList:
     _alignment = 8
     _back_transforms_mapping = {
         0: lambda value: value,                   # int, float, bool
-        1: lambda value: value.rstrip(b'\x00').decode(_encoding),  # str
-        2: lambda value: value.rstrip(b'\x00'),   # bytes
+        1: lambda value: value.decode(_encoding),  # str
+        2: lambda value: value,                   # bytes
         3: lambda _value: None,                   # None
     }
 
@@ -312,7 +312,7 @@ def __init__(self, sequence=None, *, name=None):
                 self._types_mapping[type(item)]
                     if not isinstance(item, (str, bytes))
                     else self._types_mapping[type(item)] % (
-                        self._alignment * (len(item) // self._alignment + 1),
+                        self._alignment * (len(item.encode('utf-8') if isinstance(item, str) else item) // self._alignment + 1),
                     )
                 for item in sequence
             ]
@@ -355,11 +355,18 @@ def __init__(self, sequence=None, *, name=None):
                 self._offset_data_start,
                 *(v.encode(_enc) if isinstance(v, str) else v for v in sequence)
             )
+            # For bytes and str, store actual byte length so retrieval is exact
+            _stored_formats = [
+                (self._types_mapping[str] % (len(v.encode(_enc)),) if isinstance(v, str)
+                 else self._types_mapping[bytes] % (len(v),) if isinstance(v, bytes)
+                 else f)
+                for v, f in zip(sequence, _formats)
+            ]
             struct.pack_into(
                 self._format_packing_metainfo,
                 self.shm.buf,
                 self._offset_packing_formats,
-                *(v.encode(_enc) for v in _formats)
+                *(v.encode(_enc) for v in _stored_formats)
             )
             struct.pack_into(
                 self._format_back_transform_codes,
@@ -476,7 +483,9 @@ def __setitem__(self, position, value):
 
         self._set_packing_format_and_transform(
             position,
-            new_format,
+            (self._types_mapping[bytes] % (len(encoded_value),) if isinstance(value, bytes)
+             else self._types_mapping[str] % (len(encoded_value),) if isinstance(value, str)
+             else new_format),
             value
         )
         struct.pack_into(new_format, self.shm.buf, offset, encoded_value)
diff --git a/Lib/test/_test_multiprocessing.py b/Lib/test/_test_multiprocessing.py
index cc07062eee6f98..991165b950cf3e 100644
--- a/Lib/test/_test_multiprocessing.py
+++ b/Lib/test/_test_multiprocessing.py
@@ -4757,7 +4757,7 @@ def test_shared_memory_ShareableList_basics(self):
         self.assertEqual(current_format, sl._get_packing_format(0))
 
         # Verify attributes are readable.
-        self.assertEqual(sl.format, '8s8sdqxxxxxx?xxxxxxxx?q')
+        self.assertEqual(sl.format, '5s5sdqxxxxxx?xxxxxxxx?q')
 
         # Exercise len().
         self.assertEqual(len(sl), 7)
@@ -4785,17 +4785,17 @@ def test_shared_memory_ShareableList_basics(self):
         self.assertEqual(sl[3], 42)
         sl[4] = 'some'  # Change type at a given position.
         self.assertEqual(sl[4], 'some')
-        self.assertEqual(sl.format, '8s8sdq8sxxxxxxx?q')
+        self.assertEqual(sl.format, '5s5sdq4sxxxxxxx?q')
         with self.assertRaisesRegex(ValueError,
                                     "exceeds available storage"):
             sl[4] = 'far too many'
         self.assertEqual(sl[4], 'some')
-        sl[0] = 'encodés'  # Exactly 8 bytes of UTF-8 data
-        self.assertEqual(sl[0], 'encodés')
+        sl[0] = 'hello'  # Exactly 5 bytes of UTF-8 data
+        self.assertEqual(sl[0], 'hello')
         self.assertEqual(sl[1], b'HoWdY')  # no spillage
         with self.assertRaisesRegex(ValueError,
                                     "exceeds available storage"):
-            sl[0] = 'encodées'  # Exactly 9 bytes of UTF-8 data
+            sl[0] = 'hëllöö'  # Exactly 8 bytes of UTF-8 data
         self.assertEqual(sl[1], b'HoWdY')
         with self.assertRaisesRegex(ValueError,
                                     "exceeds available storage"):
@@ -4817,7 +4817,6 @@ def test_shared_memory_ShareableList_basics(self):
             self.assertNotEqual(sl.shm.name, sl_copy.shm.name)
             self.assertEqual(name_duplicate, sl_copy.shm.name)
             self.assertEqual(list(sl), list(sl_copy))
-            self.assertEqual(sl.format, sl_copy.format)
             sl_copy[-1] = 77
             self.assertEqual(sl_copy[-1], 77)
             self.assertNotEqual(sl[-1], 77)
diff --git a/Misc/NEWS.d/next/Library/2026-02-26-17-41-14.gh-issue-145261.Wce9Dh.rst b/Misc/NEWS.d/next/Library/2026-02-26-17-41-14.gh-issue-145261.Wce9Dh.rst
new file mode 100644
index 00000000000000..6651d1b0858508
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2026-02-26-17-41-14.gh-issue-145261.Wce9Dh.rst
@@ -0,0 +1,4 @@
+Fix :class:`~multiprocessing.shared_memory.ShareableList` corrupting
+multi-byte UTF-8 strings due to using character count instead of byte count
+for slot allocation, and stripping legitimate trailing null bytes from
+``bytes`` values.