diff --git a/changelog.d/6090.feature b/changelog.d/6090.feature
new file mode 100644
index 000000000000..a6da448a1a7a
--- /dev/null
+++ b/changelog.d/6090.feature
@@ -0,0 +1 @@
+Explicitly log when a homeserver does not have the 'trusted_key_servers' config field configured.
diff --git a/docs/sample_config.yaml b/docs/sample_config.yaml
index 61d9f09a9966..29c7514f325c 100644
--- a/docs/sample_config.yaml
+++ b/docs/sample_config.yaml
@@ -1061,6 +1061,10 @@ signing_key_path: "CONFDIR/SERVERNAME.signing.key"
 # This setting supercedes an older setting named `perspectives`. The old format
 # is still supported for backwards-compatibility, but it is deprecated.
 #
+# 'trusted_key_servers' defaults to matrix.org, but using it will generate a
+# warning on start-up. To suppress this warning, set
+# 'suppress_key_server_warning' to true.
+#
 # Options for each entry in the list include:
 #
 #    server_name: the name of the server. required.
@@ -1085,11 +1089,13 @@ signing_key_path: "CONFDIR/SERVERNAME.signing.key"
 #      "ed25519:auto": "abcdefghijklmnopqrstuvwxyzabcdefghijklmopqr"
 #  - server_name: "my_other_trusted_server.example.com"
 #
-# The default configuration is:
-#
-#trusted_key_servers:
-#  - server_name: "matrix.org"
+trusted_key_servers:
+  - server_name: "matrix.org"
+
+# Uncomment the following to disable the warning that is emitted when the
+# trusted_key_servers include 'matrix.org'. See above.
 #
+#suppress_key_server_warning: true
 
 # The signing keys to use when acting as a trusted key server. If not specified
 # defaults to the server signing key.
diff --git a/synapse/config/key.py b/synapse/config/key.py
index ba2199bcebea..f039f96e9c44 100644
--- a/synapse/config/key.py
+++ b/synapse/config/key.py
@@ -50,6 +50,33 @@
 If you are *sure* you want to do this, set 'accept_keys_insecurely' on the
 trusted_key_server configuration."""
 
+TRUSTED_KEY_SERVER_NOT_CONFIGURED_WARN = """\
+Synapse requires that a list of trusted key servers are specified in order to
+provide signing keys for other servers in the federation.
+
+This homeserver does not have a trusted key server configured in
+homeserver.yaml and will fall back to the default of 'matrix.org'.
+
+Trusted key servers should be long-lived and stable which makes matrix.org a
+good choice for many admins, but some admins may wish to choose another. To
+suppress this warning, the admin should set 'trusted_key_servers' in
+homeserver.yaml to their desired key server and 'suppress_key_server_warning'
+to 'true'.
+
+In a future release the software-defined default will be removed entirely and
+the trusted key server will be defined exclusively by the value of
+'trusted_key_servers'.
+--------------------------------------------------------------------------------"""
+
+TRUSTED_KEY_SERVER_CONFIGURED_AS_M_ORG_WARN = """\
+This server is configured to use 'matrix.org' as its trusted key server via the
+'trusted_key_servers' config option. 'matrix.org' is a good choice for a key
+server since it is long-lived, stable and trusted. However, some admins may
+wish to use another server for this purpose.
+
+To suppress this warning and continue using 'matrix.org', admins should set
+'suppress_key_server_warning' to 'true' in homeserver.yaml.
+--------------------------------------------------------------------------------"""
 
 logger = logging.getLogger(__name__)
 
@@ -85,6 +112,7 @@ def read_config(self, config, config_dir_path, **kwargs):
             config.get("key_refresh_interval", "1d")
         )
 
+        suppress_key_server_warning = config.get("suppress_key_server_warning", False)
         key_server_signing_keys_path = config.get("key_server_signing_keys_path")
         if key_server_signing_keys_path:
             self.key_server_signing_keys = self.read_signing_keys(
@@ -95,6 +123,7 @@ def read_config(self, config, config_dir_path, **kwargs):
 
         # if neither trusted_key_servers nor perspectives are given, use the default.
         if "perspectives" not in config and "trusted_key_servers" not in config:
+            logger.warn(TRUSTED_KEY_SERVER_NOT_CONFIGURED_WARN)
             key_servers = [{"server_name": "matrix.org"}]
         else:
             key_servers = config.get("trusted_key_servers", [])
@@ -108,6 +137,11 @@ def read_config(self, config, config_dir_path, **kwargs):
             # merge the 'perspectives' config into the 'trusted_key_servers' config.
             key_servers.extend(_perspectives_to_key_servers(config))
 
+            if not suppress_key_server_warning and "matrix.org" in (
+                s["server_name"] for s in key_servers
+            ):
+                logger.warning(TRUSTED_KEY_SERVER_CONFIGURED_AS_M_ORG_WARN)
+
         # list of TrustedKeyServer objects
         self.key_servers = list(
             _parse_key_servers(key_servers, self.federation_verify_certificates)
@@ -190,6 +224,10 @@ def generate_config_section(
         # This setting supercedes an older setting named `perspectives`. The old format
         # is still supported for backwards-compatibility, but it is deprecated.
         #
+        # 'trusted_key_servers' defaults to matrix.org, but using it will generate a
+        # warning on start-up. To suppress this warning, set
+        # 'suppress_key_server_warning' to true.
+        #
         # Options for each entry in the list include:
         #
         #    server_name: the name of the server. required.
@@ -214,11 +252,13 @@ def generate_config_section(
         #      "ed25519:auto": "abcdefghijklmnopqrstuvwxyzabcdefghijklmopqr"
         #  - server_name: "my_other_trusted_server.example.com"
         #
-        # The default configuration is:
-        #
-        #trusted_key_servers:
-        #  - server_name: "matrix.org"
+        trusted_key_servers:
+          - server_name: "matrix.org"
+
+        # Uncomment the following to disable the warning that is emitted when the
+        # trusted_key_servers include 'matrix.org'. See above.
         #
+        #suppress_key_server_warning: true
 
         # The signing keys to use when acting as a trusted key server. If not specified
         # defaults to the server signing key.
diff --git a/synapse/config/server.py b/synapse/config/server.py
index 7f8d315954a4..a52ce30344f0 100644
--- a/synapse/config/server.py
+++ b/synapse/config/server.py
@@ -48,6 +48,13 @@
     "to join this room."
 )
 
+METRICS_PORT_WARNING = """\
+The metrics_port configuration option is deprecated in Synapse 0.31 in favour of
+a listener. Please see
+https://github.com/matrix-org/synapse/blob/master/docs/metrics-howto.md
+on how to configure the new listener.
+--------------------------------------------------------------------------------"""
+
 
 class ServerConfig(Config):
     def read_config(self, config, **kwargs):
@@ -334,14 +341,7 @@ class LimitRemoteRoomsConfig(object):
 
         metrics_port = config.get("metrics_port")
         if metrics_port:
-            logger.warn(
-                (
-                    "The metrics_port configuration option is deprecated in Synapse 0.31 "
-                    "in favour of a listener. Please see "
-                    "http://github.com/matrix-org/synapse/blob/master/docs/metrics-howto.md"
-                    " on how to configure the new listener."
-                )
-            )
+            logger.warning(METRICS_PORT_WARNING)
 
             self.listeners.append(
                 {