diff --git a/Lib/csv.py b/Lib/csv.py
index b2aaf5fd9fa91e6..3caeaed33191962 100644
--- a/Lib/csv.py
+++ b/Lib/csv.py
@@ -264,6 +264,13 @@ class dialect(Dialect):
         dialect.quotechar = quotechar or '"'
         dialect.skipinitialspace = skipinitialspace
 
+        # A guessed dialect can be inconsistent (delimiter == quotechar);
+        # surface it as csv.Error, not a raw _csv ValueError.
+        try:
+            reader(StringIO(), dialect)
+        except ValueError as e:
+            raise Error(str(e)) from None
+
         return dialect
 
 
diff --git a/Lib/test/test_csv.py b/Lib/test/test_csv.py
index 7327c1bd5f50530..84ba569e870e736 100644
--- a/Lib/test/test_csv.py
+++ b/Lib/test/test_csv.py
@@ -1463,6 +1463,14 @@ def test_sniff(self):
         self.assertEqual(dialect.quotechar, "'")
         self.assertIs(dialect.skipinitialspace, False)
 
+    def test_sniff_delimiter_equals_quotechar(self):
+        # An inconsistent guessed dialect must surface as csv.Error.
+        sniffer = csv.Sniffer()
+        for sample in ('"', '""', "''"):
+            with self.subTest(sample=sample):
+                self.assertRaises(csv.Error, sniffer.sniff, sample)
+                self.assertRaises(csv.Error, sniffer.has_header, sample)
+
     def test_delimiters(self):
         sniffer = csv.Sniffer()
         dialect = sniffer.sniff(self.sample3)
diff --git a/Misc/NEWS.d/next/Library/2026-07-09-16-41-20.gh-issue-153634.27Fs4T.rst b/Misc/NEWS.d/next/Library/2026-07-09-16-41-20.gh-issue-153634.27Fs4T.rst
new file mode 100644
index 000000000000000..f70943b18dd73ad
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2026-07-09-16-41-20.gh-issue-153634.27Fs4T.rst
@@ -0,0 +1,4 @@
+:meth:`csv.Sniffer.sniff` now raises :exc:`csv.Error` for an inconsistent
+guessed dialect (one whose delimiter equals its quote character) instead of
+returning it. :meth:`csv.Sniffer.has_header` now raises :exc:`csv.Error`
+instead of leaking a :exc:`ValueError`.