diff --git a/Lib/test/test_io.py b/Lib/test/test_io.py
--- a/Lib/test/test_io.py
+++ b/Lib/test/test_io.py
@@ -2615,6 +2615,39 @@
         txt.write('5')
         self.assertEqual(b''.join(raw._write_stack), b'123\n45')
 
+    def test_bufio_write_through(self):
+        # Issue #21396: Python io implementation doesn't flush with
+        # write_through=True unlike C implementation
+
+        # test that buffer.flush is not called with write_through=True
+        # test that buffer.write *is* called with write_through=True
+        flush_called, write_called = [], []
+        class BufferedWriter(self.BufferedWriter):
+            def flush(self, *args, **kwargs):
+                flush_called.append(True)
+                return super().flush(*args, **kwargs)
+            def write(self, *args, **kwargs):
+                write_called.append(True)
+                return super().write(*args, **kwargs)
+
+        rawio = self.BytesIO()
+        data = b"a"
+        bufio = BufferedWriter(rawio, len(data)*2)
+        textio = self.TextIOWrapper(bufio, encoding='ascii',
+                                    write_through=True)
+        # write to the buffered io but don't overflow the buffer
+        text = data.decode('ascii')
+        textio.write(text)
+
+        self.assertFalse(flush_called)
+        self.assertTrue(write_called)
+        self.assertEqual(rawio.getvalue(), b"") # no flush
+
+        write_called = [] # reset
+        textio.write(text * 10) # total content is larger than bufio buffer
+        self.assertTrue(write_called)
+        self.assertEqual(rawio.getvalue(), data * 11) # all flushed
+
     def test_read_nonbytes(self):
         # Issue #17106
         # Crash when underlying read() returns non-bytes
diff --git a/Modules/_io/textio.c b/Modules/_io/textio.c
--- a/Modules/_io/textio.c
+++ b/Modules/_io/textio.c
@@ -1297,7 +1297,7 @@
     PyObject *b;
     Py_ssize_t textlen;
     int haslf = 0;
-    int needflush = 0;
+    int needflush = 0, text_needflush = 0;
 
     CHECK_INITIALIZED(self);
 
@@ -1331,8 +1331,8 @@
     }
 
     if (self->write_through)
-        needflush = 1;
-    else if (self->line_buffering &&
+        text_needflush = 1;
+    if (self->line_buffering &&
         (haslf ||
          PyUnicode_FindChar(text, '\r', 0, PyUnicode_GET_LENGTH(text), 1) != -1))
         needflush = 1;
@@ -1363,7 +1363,8 @@
     }
     self->pending_bytes_count += PyBytes_GET_SIZE(b);
     Py_DECREF(b);
-    if (self->pending_bytes_count > self->chunk_size || needflush) {
+    if (self->pending_bytes_count > self->chunk_size || needflush ||
+        text_needflush) {
         if (_textiowrapper_writeflush(self) < 0)
             return NULL;
     }