diff --git a/.github/workflows/sdk-platform-java-downstream_protobuf_compatibility_check_nightly.yaml b/.github/workflows/sdk-platform-java-downstream_protobuf_compatibility_check_nightly.yaml
index 858c02de00f3..9d6e40d734d6 100644
--- a/.github/workflows/sdk-platform-java-downstream_protobuf_compatibility_check_nightly.yaml
+++ b/.github/workflows/sdk-platform-java-downstream_protobuf_compatibility_check_nightly.yaml
@@ -12,44 +12,25 @@ on:
   schedule:
     - cron: '0 1 * * *' # Nightly at 1am
 
+# This job intends to test the compatibility of the Protobuf version against repos not in the
+# monorepo. As repos are migrated to the monorepo, this job will be obsolete and turned off.
 name: sdk-platform-java Downstream Protobuf Compatibility Check Nightly
-env:
-  BUILD_SUBDIR: sdk-platform-java
 jobs:
-  filter:
-    runs-on: ubuntu-latest
-    outputs:
-      library: ${{ steps.filter.outputs.library }}
-    steps:
-    - uses: actions/checkout@v4
-    - uses: dorny/paths-filter@v3
-      id: filter
-      with:
-        filters: |
-          library:
-            - 'sdk-platform-java/**'
   downstream-protobuf-test:
-    needs: filter
     # This job runs if any of the three conditions match:
     # 1. PR is raised from Release-Please (PR comes from branch: release-please--branches-main)
     # 2. Job is invoked by the nightly job (scheduled event)
     # 3. Job is manually invoked via Github UI (workflow_dispatch event)
-    if: needs.filter.outputs.library == 'true' && (github.head_ref == 'release-please--branches--main' || github.event_name == 'schedule' || github.event_name == 'workflow_dispatch')
+    if: github.head_ref == 'release-please--branches--main' || github.event_name == 'schedule' || github.event_name == 'workflow_dispatch'
     runs-on: ubuntu-22.04
     strategy:
       fail-fast: false
       matrix:
         repo:
-          - google-cloud-java
           - java-bigtable
-          - java-bigquery
           - java-firestore
           - java-pubsub
           - java-pubsublite
-          - java-spanner-jdbc
-          - java-spanner
-          - java-storage
-          - java-storage-nio
         # Default Protobuf-Java versions to use are specified here. Without this, the nightly workflow won't know
         # which values to use and would resolve to ''.
         protobuf-version: ${{ fromJSON(format('[{0}]', inputs.protobuf_runtime_versions || '"4.33.5"')) }}
@@ -70,7 +51,7 @@ jobs:
       - name: Print Protobuf-Java testing version
         run: echo "Testing with Protobuf-Java v${{ matrix.protobuf-version }}"
       - name: Perform downstream source compatibility testing
-        run: REPOS_UNDER_TEST="${{ matrix.repo }}" PROTOBUF_RUNTIME_VERSION="${{ matrix.protobuf-version }}" ./.kokoro/nightly/downstream-protobuf-source-compatibility.sh
+        run: REPOS_UNDER_TEST="${{ matrix.repo }}" PROTOBUF_RUNTIME_VERSION="${{ matrix.protobuf-version }}" ./sdk-platform-java/.kokoro/nightly/downstream-protobuf-source-compatibility.sh
       - name: Perform downstream binary compatibility testing
-        run: REPOS_UNDER_TEST="${{ matrix.repo }}" PROTOBUF_RUNTIME_VERSION="${{ matrix.protobuf-version }}" ./.kokoro/nightly/downstream-protobuf-binary-compatibility.sh
+        run: REPOS_UNDER_TEST="${{ matrix.repo }}" PROTOBUF_RUNTIME_VERSION="${{ matrix.protobuf-version }}" ./sdk-platform-java/.kokoro/nightly/downstream-protobuf-binary-compatibility.sh
 
diff --git a/sdk-platform-java/.kokoro/nightly/common.sh b/sdk-platform-java/.kokoro/nightly/common.sh
new file mode 100644
index 000000000000..589ee00bfbf7
--- /dev/null
+++ b/sdk-platform-java/.kokoro/nightly/common.sh
@@ -0,0 +1,37 @@
+#!/bin/bash
+# Copyright 2025 Google LLC
+#
+# 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.
+
+# For google-cloud-java, only test specific handwritten libraries included in the monorepo. This is to
+# help speed up the execution as building the entire repo is an expensive operation. Specify the nested
+# `google-cloud-*` path (except for grafeas as it doesn't have one) because maven -pl will only build the
+# specified folder (i.e. parent folder) and ignore all the related sub-modules inside
+google_cloud_java_handwritten_maven_args="java-grafeas,java-vertexai/google-cloud-vertexai,java-resourcemanager/google-cloud-resourcemanager,java-translate/google-cloud-translate"
+
+# Checks that the protobuf compatibility scripts provide non-empty input
+function validate_protobuf_compatibility_script_inputs {
+  # Comma-delimited list of repos to test
+  if [ -z "${REPOS_UNDER_TEST}" ]; then
+    echo "REPOS_UNDER_TEST Env Var must be set. This script expects a"
+    echo "comma-delimited list: i.e REPOS_UNDER_TEST=\"java-bigtable,java-bigquery\""
+    exit 1
+  fi
+
+  # A single version of Protobuf-Java runtime to test
+  if [ -z "${PROTOBUF_RUNTIME_VERSION}" ]; then
+    echo "PROTOBUF_RUNTIME_VERSION Env Var must be set. This script expects a single "
+    echo "Protobuf-Java runtime version i.e. PROTOBUF_RUNTIME_VERSION=\"4.28.3\""
+    exit 1
+  fi
+}
\ No newline at end of file
diff --git a/sdk-platform-java/.kokoro/nightly/downstream-protobuf-binary-compatibility.sh b/sdk-platform-java/.kokoro/nightly/downstream-protobuf-binary-compatibility.sh
new file mode 100755
index 000000000000..5877eb614c73
--- /dev/null
+++ b/sdk-platform-java/.kokoro/nightly/downstream-protobuf-binary-compatibility.sh
@@ -0,0 +1,105 @@
+#!/bin/bash
+# Copyright 2025 Google LLC
+#
+# 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.
+
+set -eo pipefail
+
+scriptDir=$(realpath "$(dirname "${BASH_SOURCE[0]}")")
+source "${scriptDir}/common.sh"
+
+validate_protobuf_compatibility_script_inputs
+
+# Declare a map of downstream handwritten libraries and the relevant artifacts to test. The map stores a
+# K/V pairing of (Key: repo name, Value: comma separate list of Group ID:Artifact ID pairings). Note: The
+# value list doesn't hold the version and this needs to be parsed from the repo's versions.txt file
+declare -A repo_linkage_checker_arguments
+repo_linkage_checker_arguments["google-cloud-java"]="io.grafeas:grafeas,com.google.cloud:google-cloud-vertexai,com.google.cloud:google-cloud-resourcemanager,com.google.cloud:google-cloud-translate,com.google.api.grpc:grpc-google-cloud-vertexai-v1,com.google.api.grpc:grpc-google-cloud-vertexai-v1beta1,com.google.api.grpc:grpc-google-cloud-resourcemanager-v3,com.google.api.grpc:grpc-google-cloud-translate-v3,com.google.api.grpc:grpc-google-cloud-translate-v3beta1"
+repo_linkage_checker_arguments["java-bigtable"]="com.google.cloud:google-cloud-bigtable,com.google.api.grpc:grpc-google-cloud-bigtable-admin-v2,com.google.api.grpc:grpc-google-cloud-bigtable-v2"
+repo_linkage_checker_arguments["java-bigquery"]="com.google.cloud:google-cloud-bigquery"
+repo_linkage_checker_arguments["java-bigquerystorage"]="com.google.cloud:google-cloud-bigquerystorage,com.google.api.grpc:grpc-google-cloud-bigquerystorage-v1beta1,com.google.api.grpc:grpc-google-cloud-bigquerystorage-v1beta2,com.google.api.grpc:grpc-google-cloud-bigquerystorage-v1,com.google.api.grpc:grpc-google-cloud-bigquerystorage-v1alpha"
+repo_linkage_checker_arguments["java-datastore"]="com.google.cloud:google-cloud-datastore,com.google.cloud.datastre:datastore-v1-proto-client,com.google.api.grpc:grpc-google-cloud-datastore-admin-v1"
+repo_linkage_checker_arguments["java-firestore"]="com.google.cloud:google-cloud-firestore,com.google.cloud:google-cloud-firestore-admin,com.google.api.grpc:grpc-google-cloud-firestore-admin-v1,com.google.api.grpc:grpc-google-cloud-firestore-v1"
+repo_linkage_checker_arguments["java-logging"]="com.google.cloud:google-cloud-logging,com.google.api.grpc:grpc-google-cloud-logging-v2"
+repo_linkage_checker_arguments["java-logging-logback"]="com.google.cloud:google-cloud-logging-logback"
+repo_linkage_checker_arguments["java-pubsub"]="com.google.cloud:google-cloud-pubsub,com.google.api.grpc:grpc-google-cloud-pubsub-v1"
+repo_linkage_checker_arguments["java-pubsublite"]="com.google.cloud:google-cloud-pubsublite,com.google.api.grpc:grpc-google-cloud-pubsublite-v1"
+repo_linkage_checker_arguments["java-spanner-jdbc"]="com.google.cloud:google-cloud-spanner-jdbc"
+repo_linkage_checker_arguments["java-spanner"]="com.google.cloud:google-cloud-spanner,com.google.cloud:google-cloud-spanner-executor,com.google.api.grpc:grpc-google-cloud-spanner-v1,com.google.api.grpc:grpc-google-cloud-spanner-admin-instance-v1,com.google.api.grpc:grpc-google-cloud-spanner-admin-database-v1,com.google.api.grpc:grpc-google-cloud-spanner-executor-v1"
+repo_linkage_checker_arguments["java-storage"]="com.google.cloud:google-cloud-storage,com.google.api.grpc:gapic-google-cloud-storage-v2,com.google.api.grpc:grpc-google-cloud-storage-v2,com.google.cloud:google-cloud-storage-control,com.google.api.grpc:grpc-google-cloud-storage-control-v2"
+repo_linkage_checker_arguments["java-storage-nio"]="com.google.cloud:google-cloud-nio"
+
+# This function requires access to the versions.txt to retrieve the versions for the artifacts
+# It will try to match the artifact_id in the versions.txt file and attach it to form the GAV
+# The GAV list is required by Linkage Checker as program arguments
+function build_program_arguments() {
+  artifact_list="${repo_linkage_checker_arguments[$1]}"
+
+  for artifact in ${artifact_list//,/ }; do # Split on comma
+    artifact_id=$(echo "${artifact}" | cut -d ':' -f2)
+
+    # The grep query tries to match `{artifact_id}:{released_version}:{current_version}`.
+    # The artifact_id must be exact otherwise multiple entries may match
+    version=$(cat "versions.txt" | grep -E "^${artifact_id}:.*:.*$" | cut -d ':' -f3)
+    repo_gav_coordinate="${artifact}:${version}"
+
+    # The first entry added is not separated with a comma. Avoids generating `,{ARTIFACT_LIST}`
+    if [ -z "${linkage_checker_arguments}" ]; then
+      linkage_checker_arguments="${repo_gav_coordinate}"
+    else
+      linkage_checker_arguments="${linkage_checker_arguments},${repo_gav_coordinate}"
+    fi
+  done
+}
+
+# TODO(https://github.com/GoogleCloudPlatform/cloud-opensource-java/issues/2395): Java 17+ support for Linkage Checker
+# cloud-opensource-java contains the Linkage Checker tool
+git clone https://github.com/GoogleCloudPlatform/cloud-opensource-java.git
+pushd cloud-opensource-java
+mvn -B -ntp clean compile -T 1C
+# Linkage Checker tool resides in the /dependencies subfolder
+pushd dependencies
+
+# REPOS_UNDER_TEST Env Var accepts a comma separated list of googleapis repos to test. For Github CI,
+# this will be a single repo as Github will build a matrix of repos with each repo being tested in parallel.
+# For local invocation, you can pass a list of repos to test multiple repos together.
+for repo in ${REPOS_UNDER_TEST//,/ }; do # Split on comma
+  # Perform testing on main (with latest changes). Shallow copy as history is not important
+  git clone "https://github.com/googleapis/${repo}.git" --depth=1
+  pushd "${repo}"
+
+  if [ "${repo}" == "google-cloud-java" ]; then
+    # The `-am` command also builds anything these libraries depend on (i.e. proto-* and grpc-* sub modules)
+    mvn clean install -B -V -ntp -T 1C -DskipTests -Dclirr.skip -Denforcer.skip -Dmaven.javadoc.skip \
+      -pl "${google_cloud_java_handwritten_maven_args}" -am
+  else
+    # Install all repo modules to ~/.m2 (there can be multiple relevant artifacts to test i.e. core, admin, control)
+    mvn clean install -B -V -ntp -T 1C -DskipTests -Dclirr.skip -Denforcer.skip -Dmaven.javadoc.skip
+  fi
+
+
+  linkage_checker_arguments=""
+  build_program_arguments "${repo}"
+
+  # Linkage Checker /dependencies
+  popd
+
+  echo "Artifact List: ${linkage_checker_arguments}"
+  # The `-s` argument filters the linkage check problems that stem from the artifact
+  program_args="-r --artifacts ${linkage_checker_arguments},com.google.protobuf:protobuf-java:${PROTOBUF_RUNTIME_VERSION},com.google.protobuf:protobuf-java-util:${PROTOBUF_RUNTIME_VERSION} -s ${linkage_checker_arguments}"
+  echo "Running Linkage Checker on the repo's handwritten modules"
+  echo "Linkage Checker Program Arguments: ${program_args}"
+  mvn -B -ntp exec:java -Dexec.args="${program_args}" -P exec-linkage-checker
+done
+popd
+popd
diff --git a/sdk-platform-java/.kokoro/nightly/downstream-protobuf-source-compatibility.sh b/sdk-platform-java/.kokoro/nightly/downstream-protobuf-source-compatibility.sh
new file mode 100755
index 000000000000..aaa65b82087e
--- /dev/null
+++ b/sdk-platform-java/.kokoro/nightly/downstream-protobuf-source-compatibility.sh
@@ -0,0 +1,67 @@
+#!/bin/bash
+# Copyright 2023 Google LLC
+#
+# 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.
+
+set -eo pipefail
+
+scriptDir=$(realpath "$(dirname "${BASH_SOURCE[0]}")")
+source "${scriptDir}/common.sh"
+
+validate_protobuf_compatibility_script_inputs
+
+# REPOS_UNDER_TEST Env Var accepts a comma separated list of googleapis repos to test. For Github CI,
+# this will be a single repo as Github will build a matrix of repos with each repo being tested in parallel.
+# For local invocation, you can pass a list of repos to test multiple repos together.
+for repo in ${REPOS_UNDER_TEST//,/ }; do # Split on comma
+  # Perform source-compatibility testing on main (latest changes)
+  git clone "https://github.com/googleapis/$repo.git" --depth=1
+  pushd "$repo"
+
+  # Compile with Java 11 and run the tests with Java 8 JVM
+  mvn compile -T 1C
+
+  # JAVA8_HOME is set by the GH Actions CI
+  if [ -n "${JAVA8_HOME}" ]; then
+    surefire_opt="-Djvm=${JAVA8_HOME}/bin/java"
+  else
+    # Provide a default value for local executions that don't configure JAVA8_HOME
+    surefire_opt="-Djvm=${JAVA_HOME}/bin/java"
+  fi
+
+  # Compile the Handwritten Library with the Protobuf-Java version to test source compatibility
+  # Run unit tests to help check for any behavior differences (dependant on coverage)
+  if [ "${repo}" == "google-cloud-java" ]; then
+    # The `-am` command also builds anything these libraries depend on (i.e. proto-* and grpc-* sub modules)
+    mvn test -B -V -ntp \
+      -Dclirr.skip \
+      -Denforcer.skip \
+      -Dmaven.javadoc.skip \
+      -Denforcer.skip \
+      -Dprotobuf.version=${PROTOBUF_RUNTIME_VERSION} \
+      -pl "${google_cloud_java_handwritten_maven_args}" -am \
+      "${surefire_opt}" \
+      -T 1C
+  else
+    mvn test -B -V -ntp \
+      -Dclirr.skip \
+      -Denforcer.skip \
+      -Dmaven.javadoc.skip \
+      -Denforcer.skip \
+      -Dprotobuf.version=${PROTOBUF_RUNTIME_VERSION} \
+      "${surefire_opt}" \
+      -T 1C
+  fi
+
+  popd
+done
\ No newline at end of file