diff --git a/changelog.d/18210.misc b/changelog.d/18210.misc
new file mode 100644
index 0000000000..55ecb2d6be
--- /dev/null
+++ b/changelog.d/18210.misc
@@ -0,0 +1 @@
+In scripts-dev/complement.sh, ensure that the synapse-workers & complement-synapse images are built from the local base synapse image that was just built, instead of from the latest synapse image pulled from a public registry.
diff --git a/docker/README-testing.md b/docker/README-testing.md
index db88598b8c..f0ef8ddfd9 100644
--- a/docker/README-testing.md
+++ b/docker/README-testing.md
@@ -31,14 +31,18 @@ release of Synapse, instead of your current checkout, you can skip this step. Fr
 root of the repository:
 
 ```sh
-docker build -t matrixdotorg/synapse -f docker/Dockerfile .
+docker build -t matrixdotorg/synapse:testing -f docker/Dockerfile .
 ```
 
 Next, build the workerised Synapse docker image, which is a layer over the base
-image.
+image. To test with your local checkout, pass the name of the Synapse docker image
+built in the previous step as a build argument. Otherwise, the build will pull the
+image of the latest Synapse release and use that as the base of the workers image.
+To test with a specific release of Synapse, set the `SYNAPSE_VERSION` build argument
+to the desired release version.
 
 ```sh
-docker build -t matrixdotorg/synapse-workers -f docker/Dockerfile-workers .
+docker build -t matrixdotorg/synapse-workers --build-arg FROM=matrixdotorg/synapse:testing -f docker/Dockerfile-workers .
 ```
 
 Finally, build the multi-purpose image for Complement, which is a layer over the workers image.
@@ -47,7 +51,7 @@ Finally, build the multi-purpose image for Complement, which is a layer over the
 docker build -t complement-synapse -f docker/complement/Dockerfile docker/complement
 ```
 
-This will build an image with the tag `complement-synapse`, which can be handed to
+This will build an image with the name `complement-synapse:latest`, which can be handed to
 Complement for testing via the `COMPLEMENT_BASE_IMAGE` environment variable. Refer to
 [Complement's documentation](https://github.com/matrix-org/complement/#running) for
 how to run the tests, as well as the various available command line flags.
diff --git a/scripts-dev/complement.sh b/scripts-dev/complement.sh
index 6be9177f11..4f480938ba 100755
--- a/scripts-dev/complement.sh
+++ b/scripts-dev/complement.sh
@@ -181,24 +181,38 @@ if [ -z "$skip_docker_build" ]; then
 
         # Build the base Synapse image from the local checkout
         echo_if_github "::group::Build Docker image: matrixdotorg/synapse"
-        $CONTAINER_RUNTIME build -t matrixdotorg/synapse \
-        --build-arg TEST_ONLY_SKIP_DEP_HASH_VERIFICATION \
-        --build-arg TEST_ONLY_IGNORE_POETRY_LOCKFILE \
-        -f "docker/Dockerfile" .
+
+        # Tag local builds with a dummy registry so that later builds
+        # may reference them instead of pulling from a remote registry
+        LOCAL_REGISTRY=synapse-registry.localhost
+
+        SYNAPSE_TAG=matrixdotorg/synapse
+        $CONTAINER_RUNTIME build \
+            -t "$SYNAPSE_TAG" \
+            -t "$LOCAL_REGISTRY/$SYNAPSE_TAG" \
+            --build-arg TEST_ONLY_SKIP_DEP_HASH_VERIFICATION \
+            --build-arg TEST_ONLY_IGNORE_POETRY_LOCKFILE \
+            -f "docker/Dockerfile" .
         echo_if_github "::endgroup::"
 
         # Build the workers docker image (from the base Synapse image we just built).
+        SYNAPSE_WORKERS_TAG=matrixdotorg/synapse-workers
         echo_if_github "::group::Build Docker image: matrixdotorg/synapse-workers"
-        $CONTAINER_RUNTIME build -t matrixdotorg/synapse-workers -f "docker/Dockerfile-workers" .
+        $CONTAINER_RUNTIME build \
+            -t "$SYNAPSE_WORKERS_TAG" \
+            --build-arg FROM="$LOCAL_REGISTRY/$SYNAPSE_TAG" \
+            -f "docker/Dockerfile-workers" .
         echo_if_github "::endgroup::"
 
         # Build the unified Complement image (from the worker Synapse image we just built).
+        COMPLEMENT_SYNAPSE_TAG=complement-synapse
         echo_if_github "::group::Build Docker image: complement/Dockerfile"
-        $CONTAINER_RUNTIME build -t complement-synapse \
+        $CONTAINER_RUNTIME build \
+            -t "$COMPLEMENT_SYNAPSE_TAG" \
             `# This is the tag we end up pushing to the registry (see` \
             `# .github/workflows/push_complement_image.yml) so let's just label it now` \
             `# so people can reference it by the same name locally.` \
-            -t ghcr.io/element-hq/synapse/complement-synapse \
+            -t "ghcr.io/element-hq/synapse/$COMPLEMENT_SYNAPSE_TAG" \
             -f "docker/complement/Dockerfile" "docker/complement"
         echo_if_github "::endgroup::"