diff --git a/tornado/curl_httpclient.py b/tornado/curl_httpclient.py
index 23320e4822..397c3a9752 100644
--- a/tornado/curl_httpclient.py
+++ b/tornado/curl_httpclient.py
@@ -19,6 +19,7 @@
 import functools
 import logging
 import pycurl
+import re
 import threading
 import time
 from io import BytesIO
@@ -44,6 +45,8 @@
 
 curl_log = logging.getLogger("tornado.curl_httpclient")
 
+CR_OR_LF_RE = re.compile(b"\r|\n")
+
 
 class CurlAsyncHTTPClient(AsyncHTTPClient):
     def initialize(  # type: ignore
@@ -347,14 +350,15 @@ def _curl_setup_request(
         if "Pragma" not in request.headers:
             request.headers["Pragma"] = ""
 
-        curl.setopt(
-            pycurl.HTTPHEADER,
-            [
-                b"%s: %s"
-                % (native_str(k).encode("ASCII"), native_str(v).encode("ISO8859-1"))
-                for k, v in request.headers.get_all()
-            ],
-        )
+        encoded_headers = [
+            b"%s: %s"
+            % (native_str(k).encode("ASCII"), native_str(v).encode("ISO8859-1"))
+            for k, v in request.headers.get_all()
+        ]
+        for line in encoded_headers:
+            if CR_OR_LF_RE.search(line):
+                raise ValueError("Illegal characters in header (CR or LF): %r" % line)
+        curl.setopt(pycurl.HTTPHEADER, encoded_headers)
 
         curl.setopt(
             pycurl.HEADERFUNCTION,
diff --git a/tornado/http1connection.py b/tornado/http1connection.py
index ca50e8ff55..3bdffac115 100644
--- a/tornado/http1connection.py
+++ b/tornado/http1connection.py
@@ -38,6 +38,8 @@
 
 from typing import cast, Optional, Type, Awaitable, Callable, Union, Tuple
 
+CR_OR_LF_RE = re.compile(b"\r|\n")
+
 
 class _QuietException(Exception):
     def __init__(self) -> None:
@@ -453,8 +455,8 @@ def write_headers(
         )
         lines.extend(line.encode("latin1") for line in header_lines)
         for line in lines:
-            if b"\n" in line:
-                raise ValueError("Newline in header: " + repr(line))
+            if CR_OR_LF_RE.search(line):
+                raise ValueError("Illegal characters (CR or LF) in header: %r" % line)
         future = None
         if self.stream.closed():
             future = self._write_future = Future()
diff --git a/tornado/test/httpclient_test.py b/tornado/test/httpclient_test.py
index 31a1916199..17291f8f2e 100644
--- a/tornado/test/httpclient_test.py
+++ b/tornado/test/httpclient_test.py
@@ -725,6 +725,22 @@ def test_error_after_cancel(self):
                 if el.logged_stack:
                     break
 
+    def test_header_crlf(self):
+        # Ensure that the client doesn't allow CRLF injection in headers. RFC 9112 section 2.2
+        # prohibits a bare CR specifically and "a recipient MAY recognize a single LF as a line
+        # terminator" so we check each character separately as well as the (redundant) CRLF pair.
+        for header, name in [
+            ("foo\rbar:", "cr"),
+            ("foo\nbar:", "lf"),
+            ("foo\r\nbar:", "crlf"),
+        ]:
+            with self.subTest(name=name, position="value"):
+                with self.assertRaises(ValueError):
+                    self.fetch("/hello", headers={"foo": header})
+            with self.subTest(name=name, position="key"):
+                with self.assertRaises(ValueError):
+                    self.fetch("/hello", headers={header: "foo"})
+
 
 class RequestProxyTest(unittest.TestCase):
     def test_request_set(self):