diff --git a/synapse/handlers/saml_handler.py b/synapse/handlers/saml_handler.py
index 107f97032b..32638671c9 100644
--- a/synapse/handlers/saml_handler.py
+++ b/synapse/handlers/saml_handler.py
@@ -32,6 +32,7 @@ from synapse.types import (
     mxid_localpart_allowed_characters,
 )
 from synapse.util.async_helpers import Linearizer
+from synapse.util.iterutils import chunk_seq
 
 logger = logging.getLogger(__name__)
 
@@ -132,7 +133,17 @@ class SamlHandler:
             logger.warning("SAML2 response was not signed")
             raise SynapseError(400, "SAML2 response was not signed")
 
-        logger.info("SAML2 response: %s", saml2_auth.origxml)
+        logger.debug("SAML2 response: %s", saml2_auth.origxml)
+        for assertion in saml2_auth.assertions:
+            # kibana limits the length of a log field, whereas this is all rather
+            # useful, so split it up.
+            count = 0
+            for part in chunk_seq(str(assertion), 10000):
+                logger.info(
+                    "SAML2 assertion: %s%s", "(%i)..." % (count,) if count else "", part
+                )
+                count += 1
+
         logger.info("SAML2 mapped attributes: %s", saml2_auth.ava)
 
         try:
diff --git a/synapse/util/iterutils.py b/synapse/util/iterutils.py
index c10016fbc5..06faeebe7f 100644
--- a/synapse/util/iterutils.py
+++ b/synapse/util/iterutils.py
@@ -33,3 +33,16 @@ def batch_iter(iterable: Iterable[T], size: int) -> Iterator[Tuple[T]]:
     sourceiter = iter(iterable)
     # call islice until it returns an empty tuple
     return iter(lambda: tuple(islice(sourceiter, size)), ())
+
+
+ISeq = TypeVar("ISeq", bound=Sequence, covariant=True)
+
+
+def chunk_seq(iseq: ISeq, maxlen: int) -> Iterable[ISeq]:
+    """Split the given sequence into chunks of the given size
+
+    The last chunk may be shorter than the given size.
+
+    If the input is empty, no chunks are returned.
+    """
+    return (iseq[i : i + maxlen] for i in range(0, len(iseq), maxlen))
diff --git a/tests/util/test_itertools.py b/tests/util/test_itertools.py
new file mode 100644
index 0000000000..0ab0a91483
--- /dev/null
+++ b/tests/util/test_itertools.py
@@ -0,0 +1,47 @@
+# -*- coding: utf-8 -*-
+# Copyright 2020 The Matrix.org Foundation C.I.C.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+from synapse.util.iterutils import chunk_seq
+
+from tests.unittest import TestCase
+
+
+class ChunkSeqTests(TestCase):
+    def test_short_seq(self):
+        parts = chunk_seq("123", 8)
+
+        self.assertEqual(
+            list(parts), ["123"],
+        )
+
+    def test_long_seq(self):
+        parts = chunk_seq("abcdefghijklmnop", 8)
+
+        self.assertEqual(
+            list(parts), ["abcdefgh", "ijklmnop"],
+        )
+
+    def test_uneven_parts(self):
+        parts = chunk_seq("abcdefghijklmnop", 5)
+
+        self.assertEqual(
+            list(parts), ["abcde", "fghij", "klmno", "p"],
+        )
+
+    def test_empty_input(self):
+        parts = chunk_seq([], 5)
+
+        self.assertEqual(
+            list(parts), [],
+        )