diff --git a/CHANGES b/CHANGES
index 99f1a420060..fb7bbc21cc1 100644
--- a/CHANGES
+++ b/CHANGES
@@ -32,6 +32,8 @@ Bugs fixed
   the :dudir:`include` directive.
 * 11620: Add a new :event:`include-read` for observing and transforming
   the content of included files via the :dudir:`include` directive.
+* #11627: Restore support for copyright lines of the form ``YYYY``
+  when ``SOURCE_DATE_EPOCH`` is set.
 
 Testing
 -------
diff --git a/sphinx/config.py b/sphinx/config.py
index 598ec644331..405ca5e8deb 100644
--- a/sphinx/config.py
+++ b/sphinx/config.py
@@ -446,6 +446,7 @@ def _substitute_copyright_year(copyright_line: str, replace_year: str) -> str:
 
     Legal formats are:
 
+    * ``YYYY``
     * ``YYYY,``
     * ``YYYY ``
     * ``YYYY-YYYY,``
@@ -453,10 +454,10 @@ def _substitute_copyright_year(copyright_line: str, replace_year: str) -> str:
 
     The final year in the string is replaced with ``replace_year``.
     """
-    if not copyright_line[:4].isdigit():
+    if len(copyright_line) < 4 or not copyright_line[:4].isdigit():
         return copyright_line
 
-    if copyright_line[4] in ' ,':
+    if copyright_line[4:5] in {'', ' ', ','}:
         return replace_year + copyright_line[4:]
 
     if copyright_line[4] != '-':
diff --git a/tests/roots/test-copyright-multiline/conf.py b/tests/roots/test-copyright-multiline/conf.py
index 24bd0c5291f..a2b7b680100 100644
--- a/tests/roots/test-copyright-multiline/conf.py
+++ b/tests/roots/test-copyright-multiline/conf.py
@@ -1,4 +1,5 @@
 copyright = (
+    '2006',
     '2006-2009, Alice',
     '2010-2013, Bob',
     '2014-2017, Charlie',
diff --git a/tests/test_config.py b/tests/test_config.py
index 41b737e01b0..0be0a588cfb 100644
--- a/tests/test_config.py
+++ b/tests/test_config.py
@@ -471,6 +471,7 @@ def test_multi_line_copyright(source_date_year, app, monkeypatch):
 
     if source_date_year is None:
         # check the copyright footer line by line (empty lines ignored)
+        assert '  &#169; Copyright 2006.<br/>\n' in content
         assert '  &#169; Copyright 2006-2009, Alice.<br/>\n' in content
         assert '  &#169; Copyright 2010-2013, Bob.<br/>\n' in content
         assert '  &#169; Copyright 2014-2017, Charlie.<br/>\n' in content
@@ -479,6 +480,8 @@ def test_multi_line_copyright(source_date_year, app, monkeypatch):
 
         # check the raw copyright footer block (empty lines included)
         assert (
+            '      &#169; Copyright 2006.<br/>\n'
+            '    \n'
             '      &#169; Copyright 2006-2009, Alice.<br/>\n'
             '    \n'
             '      &#169; Copyright 2010-2013, Bob.<br/>\n'
@@ -491,6 +494,7 @@ def test_multi_line_copyright(source_date_year, app, monkeypatch):
         ) in content
     else:
         # check the copyright footer line by line (empty lines ignored)
+        assert f'  &#169; Copyright {source_date_year}.<br/>\n' in content
         assert f'  &#169; Copyright 2006-{source_date_year}, Alice.<br/>\n' in content
         assert f'  &#169; Copyright 2010-{source_date_year}, Bob.<br/>\n' in content
         assert f'  &#169; Copyright 2014-{source_date_year}, Charlie.<br/>\n' in content
@@ -499,6 +503,8 @@ def test_multi_line_copyright(source_date_year, app, monkeypatch):
 
         # check the raw copyright footer block (empty lines included)
         assert (
+            f'      &#169; Copyright {source_date_year}.<br/>\n'
+            f'    \n'
             f'      &#169; Copyright 2006-{source_date_year}, Alice.<br/>\n'
             f'    \n'
             f'      &#169; Copyright 2010-{source_date_year}, Bob.<br/>\n'