diff --git a/lib/feed_reader/formats/rss.ex b/lib/feed_reader/formats/rss.ex
index 4cd6242..bc56bc6 100644
--- a/lib/feed_reader/formats/rss.ex
+++ b/lib/feed_reader/formats/rss.ex
@@ -1,5 +1,5 @@
 defmodule FeedReader.Formats.RSS do
-  @moduledoc "RSS 2.0 format detection and normalization."
+  @moduledoc "RSS format detection and normalization."
 
   @behaviour FeedReader.Format
 
@@ -7,7 +7,17 @@ defmodule FeedReader.Formats.RSS do
 
   @impl true
   def matches_prefix?(prefix) do
-    Regex.match?(~r/<rss\b[^>]*\bversion\s*=\s*(["'])(?:0\.9(?:1|2)?|2\.0)\1/is, prefix)
+    case Regex.run(~r/<rss\b([^>]*)>/is, prefix, capture: :all_but_first) do
+      [attributes] ->
+        not Regex.match?(~r/\bversion\s*=/is, attributes) or
+          Regex.match?(
+            ~r/\bversion\s*=\s*(["'])(?:0\.9(?:1|2)?|2\.0)\1/is,
+            attributes
+          )
+
+      _none ->
+        false
+    end
   end
 
   @impl true
diff --git a/test/feed_reader/format_detection_test.exs b/test/feed_reader/format_detection_test.exs
index 195e7b4..f922cb8 100644
--- a/test/feed_reader/format_detection_test.exs
+++ b/test/feed_reader/format_detection_test.exs
@@ -6,6 +6,8 @@ defmodule FeedReader.FormatDetectionTest do
   test "detects RSS, Atom, and RDF from a bounded XML prefix" do
     assert FeedReader.detect_format("<?xml version='1.0'?><rss version='2.0'>") == :rss
     assert FeedReader.detect_format("<rss version='0.91'>") == :rss
+    assert FeedReader.detect_format("<rss xmlns:xsi='urn:test'>") == :rss
+    assert FeedReader.detect_format("<rss version='1.0'>") == :unknown
     assert FeedReader.detect_format("<feed xmlns='http://www.w3.org/2005/Atom'>") == :atom
     assert FeedReader.detect_format("<rdf:RDF xmlns:rdf='urn:rdf'>") == :rdf
     assert FeedReader.detect_format("<html><body>not a feed</body></html>") == :unknown