diff --git a/CHANGES/13686.doc.rst b/CHANGES/13686.doc.rst
new file mode 100644
index 00000000000..dfdda30ed4c
--- /dev/null
+++ b/CHANGES/13686.doc.rst
@@ -0,0 +1,3 @@
+Corrected the documented signature of :py:meth:`~aiohttp.StreamReader.readuntil`, which
+showed a ``str`` separator although the method takes ``bytes``, and documented its
+keyword-only ``max_size`` argument -- by :user:`hxperl`.
diff --git a/CONTRIBUTORS.txt b/CONTRIBUTORS.txt
index fec05a6644a..df269e16d02 100644
--- a/CONTRIBUTORS.txt
+++ b/CONTRIBUTORS.txt
@@ -157,6 +157,7 @@ Gary Leung
 Gary Wilson Jr.
 Gene Hoffman
 Gennady Andreyev
+Geonseok Lee
 George Fifth
 Georges Dubus
 goingforstudying-ctrl
diff --git a/docs/streams.rst b/docs/streams.rst
index c74a0c1ad93..cbbcf44d751 100644
--- a/docs/streams.rst
+++ b/docs/streams.rst
@@ -79,7 +79,7 @@ Reading Attributes and Methods
 
    :return bytes: the given line
 
-.. method:: StreamReader.readuntil(separator="\n")
+.. method:: StreamReader.readuntil(separator=b"\n", *, max_size=None)
       :async:
 
    Read until separator, where `separator` is a sequence of bytes.
@@ -92,6 +92,10 @@ Reading Attributes and Methods
 
    .. versionadded:: 3.8
 
+   :param int max_size: maximum number of bytes to buffer while searching for
+      *separator*; exceeding it raises ``LineTooLong``. ``None``, the default,
+      uses the stream's high-water limit.
+
    :return bytes: the given data
 
 .. method:: StreamReader.readchunk()