diff --git a/INSTALL.md b/INSTALL.md
index 76833e0f8c44..de6893530d26 100644
--- a/INSTALL.md
+++ b/INSTALL.md
@@ -375,9 +375,13 @@ To configure Synapse to expose an HTTPS port, you will need to edit
 * You will also need to uncomment the `tls_certificate_path` and
   `tls_private_key_path` lines under the `TLS` section. You can either
   point these settings at an existing certificate and key, or you can
-  enable Synapse's built-in ACME (Let's Encrypt) support.  Instructions
+  enable Synapse's built-in ACME (Let's Encrypt) support. Instructions
   for having Synapse automatically provision and renew federation
-  certificates through ACME can be found at [ACME.md](docs/ACME.md).
+  certificates through ACME can be found at [ACME.md](docs/ACME.md). If you
+  are using your own certificate, be sure to use a `.pem` file that includes
+  the full certificate chain including any intermediate certificates (for
+  instance, if using certbot, use `fullchain.pem` as your certificate, not
+  `cert.pem`).
 
 For those of you upgrading your TLS certificate in readiness for Synapse 1.0,
 please take a look at `our guide <docs/MSC1711_certificates_FAQ.md#configuring-certificates-for-compatibility-with-synapse-100>`_.
diff --git a/changelog.d/4849.misc b/changelog.d/4849.misc
new file mode 100644
index 000000000000..f2cab20b4476
--- /dev/null
+++ b/changelog.d/4849.misc
@@ -0,0 +1 @@
+Update install docs to explicitly state a full-chain (not just the top-level) TLS certificate must be provided to Synapse. This caused some people's Synapse ports to appear correct in a browser but still (rightfully so) upset the federation tester.
\ No newline at end of file
diff --git a/docs/sample_config.yaml b/docs/sample_config.yaml
index 22d5e6b1d78d..5f2534e4654e 100644
--- a/docs/sample_config.yaml
+++ b/docs/sample_config.yaml
@@ -246,6 +246,11 @@ listeners:
 # See 'ACME support' below to enable auto-provisioning this certificate via
 # Let's Encrypt.
 #
+# If supplying your own, be sure to use a `.pem` file that includes the
+# full certificate chain including any intermediate certificates (for
+# instance, if using certbot, use `fullchain.pem` as your certificate,
+# not `cert.pem`).
+#
 #tls_certificate_path: "CONFDIR/SERVERNAME.tls.crt"
 
 # PEM-encoded private key for TLS
diff --git a/synapse/config/tls.py b/synapse/config/tls.py
index 40045de7ace5..f0014902da72 100644
--- a/synapse/config/tls.py
+++ b/synapse/config/tls.py
@@ -181,6 +181,11 @@ def default_config(self, config_dir_path, server_name, **kwargs):
         # See 'ACME support' below to enable auto-provisioning this certificate via
         # Let's Encrypt.
         #
+        # If supplying your own, be sure to use a `.pem` file that includes the
+        # full certificate chain including any intermediate certificates (for
+        # instance, if using certbot, use `fullchain.pem` as your certificate,
+        # not `cert.pem`).
+        #
         #tls_certificate_path: "%(tls_certificate_path)s"
 
         # PEM-encoded private key for TLS