diff --git a/httpie/output/formatters/headers.py b/httpie/output/formatters/headers.py
index 2188193902..1b701bee51 100644
--- a/httpie/output/formatters/headers.py
+++ b/httpie/output/formatters/headers.py
@@ -14,5 +14,5 @@ def format_headers(self, headers: str) -> str:
 
         """
         lines = headers.splitlines()
-        headers = sorted(lines[1:], key=lambda h: h.split(':')[0])
+        headers = sorted(lines[1:], key=lambda h: h.split(':')[0].lower())
         return '\r\n'.join(lines[:1] + headers)
diff --git a/tests/test_output.py b/tests/test_output.py
index 2242177dbc..2adf92812e 100644
--- a/tests/test_output.py
+++ b/tests/test_output.py
@@ -392,6 +392,14 @@ def get_headers(sort):
         assert f'XXX: foo{CRLF}ZZZ: foo' in r_sorted
         assert f'ZZZ: foo{CRLF}XXX: foo' in r_unsorted
 
+    def test_header_sorting_is_case_insensitive(self):
+        r = http(
+            '--offline', '--print=H',
+            '--format-options', 'headers.sort:true',
+            'example.org', 'Zebra:foo', 'apple:foo',
+        )
+        assert r.index('apple: foo') < r.index('Zebra: foo')
+
     @pytest.mark.parametrize(
         'options, expected_json',
         [