diff --git a/Lib/fileinput.py b/Lib/fileinput.py
index de2951844ae..87758ad82b3 100644
--- a/Lib/fileinput.py
+++ b/Lib/fileinput.py
@@ -320,7 +320,10 @@ def readline(self):
             self._backupfilename = 0
             if self._filename == '-':
                 self._filename = '<stdin>'
-                self._file = sys.stdin
+                if 'b' in self._mode:
+                    self._file = sys.stdin.buffer
+                else:
+                    self._file = sys.stdin
                 self._isstdin = True
             else:
                 if self._inplace:
diff --git a/Lib/test/test_fileinput.py b/Lib/test/test_fileinput.py
index eba55c99a4b..1d089f52b8a 100644
--- a/Lib/test/test_fileinput.py
+++ b/Lib/test/test_fileinput.py
@@ -19,11 +19,12 @@
 except ImportError:
     gzip = None
 
-from io import StringIO
+from io import BytesIO, StringIO
 from fileinput import FileInput, hook_encoded
 
 from test.support import verbose, TESTFN, run_unittest, check_warnings
 from test.support import unlink as safe_unlink
+from unittest import mock
 
 
 # The fileinput module has 2 interfaces: the FileInput class which does
@@ -232,6 +233,13 @@ def test_opening_mode(self):
         finally:
             remove_tempfiles(t1)
 
+    def test_stdin_binary_mode(self):
+        with mock.patch('sys.stdin') as m_stdin:
+            m_stdin.buffer = BytesIO(b'spam, bacon, sausage, and spam')
+            fi = FileInput(files=['-'], mode='rb')
+            lines = list(fi)
+            self.assertEqual(lines, [b'spam, bacon, sausage, and spam'])
+
     def test_file_opening_hook(self):
         try:
             # cannot use openhook and inplace mode
diff --git a/Misc/ACKS b/Misc/ACKS
index bbcd00e7552..62d2b204a13 100644
--- a/Misc/ACKS
+++ b/Misc/ACKS
@@ -674,6 +674,7 @@ Mads Kiilerich
 Jason Killen
 Jan Kim
 Taek Joo Kim
+Sam Kimbrel
 W. Trevor King
 Paul Kippes
 Steve Kirsch
diff --git a/Misc/NEWS b/Misc/NEWS
index eff6573d947..161016ab402 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -23,6 +23,9 @@ Core and Builtins
 Library
 -------
 
+- Issue #21075: fileinput.FileInput now reads bytes from standard stream if
+  binary mode is specified.  Patch by Sam Kimbrel.
+
 - Issue #21396: Fix TextIOWrapper(..., write_through=True) to not force a
   flush() on the underlying binary stream.  Patch by akira.