diff --git a/.circleci/config.yml b/.circleci/config.yml
index e03f01b83703..5266544f3c37 100644
--- a/.circleci/config.yml
+++ b/.circleci/config.yml
@@ -9,6 +9,8 @@ jobs:
       - store_artifacts:
           path: ~/project/logs
           destination: logs
+      - store_test_results:
+          path: logs
   sytestpy2postgres:
     machine: true
     steps:
@@ -18,6 +20,34 @@ jobs:
       - store_artifacts:
           path: ~/project/logs
           destination: logs
+      - store_test_results:
+          path: logs
+  sytestpy2merged:
+    machine: true
+    steps:
+      - checkout
+      - run: bash .circleci/merge_base_branch.sh
+      - run: docker pull matrixdotorg/sytest-synapsepy2
+      - run: docker run --rm -it -v $(pwd)\:/src -v $(pwd)/logs\:/logs matrixdotorg/sytest-synapsepy2
+      - store_artifacts:
+          path: ~/project/logs
+          destination: logs
+      - store_test_results:
+          path: logs
+
+  sytestpy2postgresmerged:
+    machine: true
+    steps:
+      - checkout
+      - run: bash .circleci/merge_base_branch.sh
+      - run: docker pull matrixdotorg/sytest-synapsepy2
+      - run: docker run --rm -it -v $(pwd)\:/src -v $(pwd)/logs\:/logs -e POSTGRES=1 matrixdotorg/sytest-synapsepy2
+      - store_artifacts:
+          path: ~/project/logs
+          destination: logs
+      - store_test_results:
+          path: logs
+
   sytestpy3:
     machine: true
     steps:
@@ -27,6 +57,8 @@ jobs:
       - store_artifacts:
           path: ~/project/logs
           destination: logs
+      - store_test_results:
+          path: logs
   sytestpy3postgres:
     machine: true
     steps:
@@ -36,6 +68,32 @@ jobs:
       - store_artifacts:
           path: ~/project/logs
           destination: logs
+      - store_test_results:
+          path: logs
+  sytestpy3merged:
+    machine: true
+    steps:
+      - checkout
+      - run: bash .circleci/merge_base_branch.sh
+      - run: docker pull matrixdotorg/sytest-synapsepy3
+      - run: docker run --rm -it -v $(pwd)\:/src -v $(pwd)/logs\:/logs hawkowl/sytestpy3
+      - store_artifacts:
+          path: ~/project/logs
+          destination: logs
+      - store_test_results:
+          path: logs
+  sytestpy3postgresmerged:
+    machine: true
+    steps:
+      - checkout
+      - run: bash .circleci/merge_base_branch.sh
+      - run: docker pull matrixdotorg/sytest-synapsepy3
+      - run: docker run --rm -it -v $(pwd)\:/src -v $(pwd)/logs\:/logs -e POSTGRES=1 matrixdotorg/sytest-synapsepy3
+      - store_artifacts:
+          path: ~/project/logs
+          destination: logs
+      - store_test_results:
+          path: logs
 
 workflows:
   version: 2
@@ -43,6 +101,14 @@ workflows:
     jobs:
       - sytestpy2
       - sytestpy2postgres
+      - sytestpy2merged:
+          filters:
+            branches:
+              ignore: /develop|master/
+      - sytestpy2postgresmerged:
+          filters:
+            branches:
+              ignore: /develop|master/
 # Currently broken while the Python 3 port is incomplete
 #      - sytestpy3
 #      - sytestpy3postgres
diff --git a/.circleci/merge_base_branch.sh b/.circleci/merge_base_branch.sh
new file mode 100755
index 000000000000..2d700dbf1189
--- /dev/null
+++ b/.circleci/merge_base_branch.sh
@@ -0,0 +1,27 @@
+#!/usr/bin/env bash
+
+set -e
+
+# CircleCI doesn't give CIRCLE_PR_NUMBER in the environment for non-forked PRs. Wonderful.
+# In this case, we just need to do some ~shell magic~ to strip it out of the PULL_REQUEST URL.
+echo 'export CIRCLE_PR_NUMBER="${CIRCLE_PR_NUMBER:-${CIRCLE_PULL_REQUEST##*/}}"' >> "$BASH_ENV"
+source $BASH_ENV
+
+if [[ -z "${CIRCLE_PR_NUMBER}" ]]
+then
+    echo "Can't figure out what the PR number is!"
+    exit 1
+fi
+
+# Get the reference, using the GitHub API
+GITBASE=`curl -q https://api.github.com/repos/matrix-org/synapse/pulls/${CIRCLE_PR_NUMBER} | jq -r '.base.ref'`
+
+# Show what we are before
+git show -s
+
+# Fetch and merge. If it doesn't work, it will raise due to set -e.
+git fetch -u origin $GITBASE
+git merge --no-edit origin/$GITBASE
+
+# Show what we are after.
+git show -s
\ No newline at end of file
diff --git a/changelog.d/3704.misc b/changelog.d/3704.misc
new file mode 100644
index 000000000000..aaae0fbd6304
--- /dev/null
+++ b/changelog.d/3704.misc
@@ -0,0 +1 @@
+CircleCI tests now run on the potential merge of a PR.