diff --git a/hgext/convert/subversion.py b/hgext/convert/subversion.py
--- a/hgext/convert/subversion.py
+++ b/hgext/convert/subversion.py
@@ -347,6 +347,15 @@ def issvnurl(ui, url):
         path = os.path.abspath(url)
     if proto == b'file':
         path = util.pconvert(path)
+    elif proto in (b'http', 'https'):
+        if not encoding.isasciistr(path):
+            ui.warn(
+                _(
+                    b"Subversion sources don't support non-ASCII characters in "
+                    b"HTTP(S) URLs. Please percent-encode them.\n"
+                )
+            )
+            return False
     check = protomap.get(proto, lambda *args: False)
     while b'/' in path:
         if check(ui, path, proto):
diff --git a/tests/test-convert-svn-encoding.t b/tests/test-convert-svn-encoding.t
--- a/tests/test-convert-svn-encoding.t
+++ b/tests/test-convert-svn-encoding.t
@@ -153,6 +153,16 @@ Check tags are in UTF-8
 
   $ cd ..
 
+Subversion sources don't support non-ASCII characters in HTTP(S) URLs.
+
+  $ XFF=$($PYTHON -c 'from mercurial.utils.procutil import stdout; stdout.write(b"\xff")')
+  $ hg convert --source-type=svn http://localhost:$HGPORT/$XFF test
+  initializing destination test repository
+  Subversion sources don't support non-ASCII characters in HTTP(S) URLs. Please percent-encode them.
+  http://localhost:$HGPORT/\xff does not look like a Subversion repository (esc)
+  abort: http://localhost:$HGPORT/\xff: missing or unsupported repository (esc)
+  [255]
+
 #if py3
 For now, on Python 3, we abort when encountering non-UTF-8 percent-encoded
 bytes in a filename.