diff --git a/.github/workflows/smoke-tests.yaml b/.github/workflows/smoke-tests.yaml
index 181c8f7..ecfce17 100644
--- a/.github/workflows/smoke-tests.yaml
+++ b/.github/workflows/smoke-tests.yaml
@@ -183,3 +183,126 @@ jobs:
           split-total: 5
           split-index: 1
       - run: echo "The output test suite is ${{ steps.split-tests.outputs.test-suite }}"
+
+  smoke-test-working-directory:
+    runs-on: ubuntu-latest
+    name: Smoke Test Working Directory
+    steps:
+      - uses: actions/checkout@v7
+      - uses: actions/checkout@v7
+        with:
+          repository: eliotsykes/rspec-rails-examples
+          path: examples/
+      - uses: ./
+        id: relative
+        name: Relative working-directory
+        with:
+          working-directory: examples
+          glob: spec/**/*_spec.rb
+          split-total: 1
+          split-index: 0
+      - uses: ./
+        id: absolute
+        name: Absolute working-directory
+        with:
+          working-directory: ${{ github.workspace }}/examples
+          glob: spec/**/*_spec.rb
+          split-total: 1
+          split-index: 0
+      - name: Assert output is relative to working-directory and covers the glob
+        env:
+          RELATIVE: ${{ steps.relative.outputs.test-suite }}
+          ABSOLUTE: ${{ steps.absolute.outputs.test-suite }}
+        run: |
+          set -euo pipefail
+
+          echo "$RELATIVE" | tr ' ' '\n' | sed '/^$/d' | sort -u > /tmp/relative.txt
+          echo "$ABSOLUTE" | tr ' ' '\n' | sed '/^$/d' | sort -u > /tmp/absolute.txt
+          (cd examples && find spec -name '*_spec.rb' | sort -u) > /tmp/expected.txt
+
+          if [ ! -s /tmp/expected.txt ]; then
+            echo "::error::Fixture has no spec files, the test is not testing anything"
+            exit 1
+          fi
+
+          if ! diff -u /tmp/expected.txt /tmp/relative.txt; then
+            echo "::error::Relative working-directory output does not match the files under examples/spec"
+            exit 1
+          fi
+
+          if ! diff -u /tmp/relative.txt /tmp/absolute.txt; then
+            echo "::error::Absolute and relative working-directory outputs differ"
+            exit 1
+          fi
+
+          if grep -q '^examples/' /tmp/relative.txt; then
+            echo "::error::Output paths should be relative to working-directory, not the workspace"
+            exit 1
+          fi
+
+          echo "OK: $(wc -l < /tmp/expected.txt) files, paths relative to working-directory"
+
+  smoke-test-expected-failures:
+    runs-on: ubuntu-latest
+    name: Smoke Test Expected Failures
+    steps:
+      - uses: actions/checkout@v7
+      - uses: actions/checkout@v7
+        with:
+          repository: eliotsykes/rspec-rails-examples
+          path: examples/
+      - uses: ./
+        id: missing-working-directory
+        name: Nonexistent working-directory
+        continue-on-error: true
+        with:
+          working-directory: examples/does-not-exist
+          glob: spec/**/*_spec.rb
+          split-total: 2
+          split-index: 0
+      - uses: ./
+        id: index-out-of-range
+        name: split-index equal to split-total
+        continue-on-error: true
+        with:
+          glob: examples/spec/**/*_spec.rb
+          split-total: 2
+          split-index: 2
+      - uses: ./
+        id: zero-total
+        name: split-total of zero
+        continue-on-error: true
+        with:
+          glob: examples/spec/**/*_spec.rb
+          split-total: 0
+          split-index: 0
+      - uses: ./
+        id: control
+        name: Control, valid inputs must still pass
+        continue-on-error: true
+        with:
+          glob: examples/spec/**/*_spec.rb
+          split-total: 2
+          split-index: 1
+      - name: Assert each invalid input failed and the control passed
+        env:
+          MISSING_WORKING_DIRECTORY: ${{ steps.missing-working-directory.outcome }}
+          INDEX_OUT_OF_RANGE: ${{ steps.index-out-of-range.outcome }}
+          ZERO_TOTAL: ${{ steps.zero-total.outcome }}
+          CONTROL: ${{ steps.control.outcome }}
+        run: |
+          set -euo pipefail
+          status=0
+          expect() {
+            if [ "$2" != "$3" ]; then
+              echo "::error::$1: expected outcome '$3' but got '$2'"
+              status=1
+            else
+              echo "OK: $1 -> $2"
+            fi
+          }
+          expect "nonexistent working-directory" "$MISSING_WORKING_DIRECTORY" failure
+          expect "split-index equal to split-total" "$INDEX_OUT_OF_RANGE" failure
+          expect "split-total of zero" "$ZERO_TOTAL" failure
+          expect "valid inputs (control)" "$CONTROL" success
+          exit "$status"
diff --git a/README.md b/README.md
index 4f1b379..19cae61 100644
--- a/README.md
+++ b/README.md
@@ -58,6 +58,7 @@ Timings must be per test **file**. If your runner writes one report per worker,
 | `exclude-glob` | Glob pattern to exclude test files                            | `string`  |          | `""`                |
 | `junit-path`   | Path to a JUnit XML report to use for test timings            | `string`  |          | `""`                |
 | `line-count`   | Use line count to estimate test duration                      | `bool`    |          | `false`             |
+| `working-directory` | Directory to run in, relative to `$GITHUB_WORKSPACE` (or absolute). `glob`, `exclude-glob` and `junit-path` are resolved relative to it, and `test-suite` paths are relative to it. Useful for monorepos | `string` |  | `""` (the workspace) |
 
 ### Outputs
 
diff --git a/action.yml b/action.yml
index 8513071..d9192f5 100644
--- a/action.yml
+++ b/action.yml
@@ -22,6 +22,10 @@ inputs:
   line-count:
     description: Use line count to estimate test times (boolean)
     required: false
+  working-directory:
+    description: Directory to run in, relative to GITHUB_WORKSPACE (or absolute). The glob, exclude-glob and junit-path inputs are resolved relative to it, and the output paths are relative to it. Defaults to GITHUB_WORKSPACE.
+    required: false
+    default: ""
 outputs:
   test-suite:
     description: "A subset of tests, based on the the split index and split type"
@@ -46,6 +50,9 @@ runs:
         SPLIT_TESTS_SHA256: 6988628a956f065c9eced4bb77bcc0b5c111717c1d4748b701c03afabb8349dd
       run: |
         set -euo pipefail
+        # Install into GITHUB_WORKSPACE so the binary can be found by absolute
+        # path regardless of the working-directory input.
+        cd "$GITHUB_WORKSPACE"
         curl -fL "https://github.com/leonid-shevtsov/split_tests/releases/download/${SPLIT_TESTS_VERSION}/split_tests.linux.gz" -o split_tests.gz
         if ! echo "${SPLIT_TESTS_SHA256}  split_tests.gz" | sha256sum -c -; then
           echo "::error::split_tests ${SPLIT_TESTS_VERSION} checksum mismatch. Actual sha256: $(sha256sum split_tests.gz | cut -d' ' -f1)" >&2
@@ -63,9 +70,12 @@ runs:
         EXCLUDE_GLOB_INPUT: ${{ inputs.exclude-glob }}
         JUNIT_PATH: ${{ inputs.junit-path }}
         LINE_COUNT: ${{ inputs.line-count }}
+        WORKING_DIRECTORY: ${{ inputs.working-directory }}
       run: |
         set -euo pipefail
 
+        SPLIT_TESTS_BIN="$GITHUB_WORKSPACE/split_tests"
+
         # setup split type
         if [ -n "$JUNIT_PATH" ]; then
           echo "junit-path set. Splitting based on JUnit timings"
@@ -83,7 +93,17 @@ runs:
           EXCLUDE_GLOB=(-exclude-glob="$EXCLUDE_GLOB_INPUT")
         fi
 
-        TESTS=$(./split_tests "${SPLIT_BY[@]}" -split-index="$SPLIT_INDEX" -split-total="$SPLIT_TOTAL" -glob="$GLOB" "${EXCLUDE_GLOB[@]}")
+        # Resolve relative paths from the workspace, then run from there.
+        cd "$GITHUB_WORKSPACE"
+        if [ -n "$WORKING_DIRECTORY" ]; then
+          if [ ! -d "$WORKING_DIRECTORY" ]; then
+            echo "::error::working-directory '$WORKING_DIRECTORY' does not exist"
+            exit 1
+          fi
+          cd "$WORKING_DIRECTORY"
+        fi
+
+        TESTS=$("$SPLIT_TESTS_BIN" "${SPLIT_BY[@]}" -split-index="$SPLIT_INDEX" -split-total="$SPLIT_TOTAL" -glob="$GLOB" "${EXCLUDE_GLOB[@]}")
 
         echo "$TESTS"