diff --git a/Lib/test/test_threading.py b/Lib/test/test_threading.py
index e468fcd7601c15..a5b2bc8fe75be1 100644
--- a/Lib/test/test_threading.py
+++ b/Lib/test/test_threading.py
@@ -2230,6 +2230,9 @@ def test__all__(self):
 @unittest.skipUnless(hasattr(_thread, 'set_name'), "missing _thread.set_name")
 @unittest.skipUnless(hasattr(_thread, '_get_name'), "missing _thread._get_name")
 def test_set_name(self):
+ # Ensure main thread name is restored after test
+ self.addCleanup(_thread.set_name, _thread._get_name())
+
 # set_name() limit in bytes
 truncate = getattr(_thread, "_NAME_MAXLEN", None)
 limit = truncate or 100
@@ -2269,7 +2272,8 @@ def test_set_name(self):
 tests.append(os_helper.TESTFN_UNENCODABLE)
 
 if sys.platform.startswith("sunos"):
- encoding = "utf-8"
+ # Use ASCII encoding on Solaris/Illumos/OpenIndiana
+ encoding = "ascii"
 else:
 encoding = sys.getfilesystemencoding()
 
@@ -2285,7 +2289,7 @@ def work():
 if truncate is not None:
 encoded = encoded[:truncate]
 if sys.platform.startswith("sunos"):
- expected = encoded.decode("utf-8", "surrogateescape")
+ expected = encoded.decode("ascii", "surrogateescape")
 else:
 expected = os.fsdecode(encoded)
 else:
@@ -2304,7 +2308,11 @@ def work():
 if '0円' in expected:
 expected = expected.split('0円', 1)[0]
 
- with self.subTest(name=name, expected=expected):
+ with self.subTest(name=name, expected=expected, thread="main"):
+ _thread.set_name(name)
+ self.assertEqual(_thread._get_name(), expected)
+
+ with self.subTest(name=name, expected=expected, thread="worker"):
 work_name = None
 thread = threading.Thread(target=work, name=name)
 thread.start()
diff --git a/Misc/ACKS b/Misc/ACKS
index 83bc62726eecc9..94d7948f1b58fd 100644
--- a/Misc/ACKS
+++ b/Misc/ACKS
@@ -482,6 +482,7 @@ Weilin Du
 John DuBois
 Paul Dubois
 Jacques Ducasse
+Jadon Duff
 Andrei Dorian Duma
 Graham Dumpleton
 Quinn Dunkan
diff --git a/Misc/NEWS.d/next/Core_and_Builtins/2025-08-21-06-31-42.gh-issue-138004.FH2Hre.rst b/Misc/NEWS.d/next/Core_and_Builtins/2025-08-21-06-31-42.gh-issue-138004.FH2Hre.rst
new file mode 100644
index 00000000000000..e73be998f4be7b
--- /dev/null
+++ b/Misc/NEWS.d/next/Core_and_Builtins/2025-08-21-06-31-42.gh-issue-138004.FH2Hre.rst
@@ -0,0 +1 @@
+On Solaris/Illumos platforms, thread names are now encoded as ASCII to avoid errors on systems (e.g. OpenIndiana) that don't support non-ASCII names.
diff --git a/Modules/_threadmodule.c b/Modules/_threadmodule.c
index 775cd704cda4ed..2db5a4613486c2 100644
--- a/Modules/_threadmodule.c
+++ b/Modules/_threadmodule.c
@@ -2471,7 +2471,9 @@ _thread__get_name_impl(PyObject *module)
 }
 
 #ifdef __sun
- return PyUnicode_DecodeUTF8(name, strlen(name), "surrogateescape");
+ // gh-138004: Decode Solaris/Illumos (e.g. OpenIndiana) thread names
+ // from ASCII, since OpenIndiana only supports ASCII names.
+ return PyUnicode_DecodeASCII(name, strlen(name), "surrogateescape");
 #else
 return PyUnicode_DecodeFSDefault(name);
 #endif
@@ -2509,8 +2511,9 @@ _thread_set_name_impl(PyObject *module, PyObject *name_obj)
 {
 #ifndef MS_WINDOWS
 #ifdef __sun
- // Solaris always uses UTF-8
- const char *encoding = "utf-8";
+ // gh-138004: Encode Solaris/Illumos thread names to ASCII,
+ // since OpenIndiana does not support non-ASCII names.
+ const char *encoding = "ascii";
 #else
 // Encode the thread name to the filesystem encoding using the "replace"
 // error handler
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