diff --git a/.github/workflows/coverage.yml b/.github/workflows/coverage.yml
index 9f4cac466d..ef173a2d93 100644
--- a/.github/workflows/coverage.yml
+++ b/.github/workflows/coverage.yml
@@ -12,10 +12,17 @@ concurrency:
 
 env:
   OCAMLRUNPARAM: b
+  # When changing the setup-ocaml version, also adjust it in the setup step
+  # further below.
+  SETUP_OCAML_VERSION: 3.6.0 # OPAM <2.6.0
 
 jobs:
   coverage:
     runs-on: ubuntu-24.04-arm
+    env:
+      OS: ubuntu-24.04-arm
+      OCAML_COMPILER: 5.3.0
+      RUST_TARGET: aarch64-unknown-linux-gnu
     steps:
       - name: Checkout
         uses: actions/checkout@v6
@@ -35,6 +42,17 @@ jobs:
           packages: bubblewrap darcs g++-multilib gcc-multilib mercurial musl-tools rsync cmake
           version: v4
 
+      # --- rewatch build cache ---------------------------------------------
+      # `make coverage` shells out to `make` which builds rewatch; cache the
+      # cargo target dir so subsequent runs skip the Rust toolchain install
+      # and the cargo build when the rewatch sources haven't changed.
+      - name: Restore rewatch build cache
+        id: rewatch-build-cache
+        uses: actions/cache@v5
+        with:
+          path: rewatch/target
+          key: rewatch-build-v3-${{ env.RUST_TARGET }}-${{ hashFiles('rewatch/src/**', 'rewatch/Cargo.lock') }}
+
       - name: Determine Rust toolchain version
         id: rust-version
         run: |
@@ -42,12 +60,14 @@ jobs:
           echo "version=${version}" >> "$GITHUB_OUTPUT"
 
       - name: Install Rust toolchain
+        if: steps.rewatch-build-cache.outputs.cache-hit != 'true'
         uses: dtolnay/rust-toolchain@master
         with:
           toolchain: ${{ steps.rust-version.outputs.version }}
           components: clippy, rustfmt
 
       - name: Build rewatch
+        if: steps.rewatch-build-cache.outputs.cache-hit != 'true'
         run: cargo build --manifest-path rewatch/Cargo.toml --release
 
       - name: Copy rewatch binary
@@ -55,18 +75,126 @@ jobs:
           cp rewatch/target/release/rescript rescript
           ./scripts/copyExes.js --rewatch
 
-      - name: Use OCaml
+      # --- OPAM environment cache ------------------------------------------
+      # The OPAM install step is the dominant cost on a cold run (often >10
+      # min). Cache the whole opam tree, keyed on the OS, setup-ocaml
+      # version, compiler, and the `.opam` files. The cache also includes
+      # the dev-setup dependencies (e.g. bisect_ppx) so the key is distinct
+      # from the main CI workflow's cache.
+      - name: Get OPAM cache key
+        run: echo "opam_cache_key=opam-coverage-v1-${{ env.OS }}-${{ env.SETUP_OCAML_VERSION }}-${{ env.OCAML_COMPILER }}-${{ hashFiles('*.opam') }}" >> $GITHUB_ENV
+
+      - name: Restore OPAM environment
+        id: cache-opam-env
+        uses: actions/cache/restore@v5
+        with:
+          path: |
+            ${{ runner.tool_cache }}/opam
+            ~/.opam
+            _opam
+            .opam-path
+          key: ${{ env.opam_cache_key }}
+
+      - name: Use OCaml ${{ env.OCAML_COMPILER }}
+        if: steps.cache-opam-env.outputs.cache-hit != 'true'
         uses: ocaml/setup-ocaml@v3.6.0
         with:
-          ocaml-compiler: 5.3.0
+          ocaml-compiler: ${{ env.OCAML_COMPILER }}
           opam-pin: false
 
+      - name: Get OPAM executable path
+        if: steps.cache-opam-env.outputs.cache-hit != 'true'
+        uses: actions/github-script@v9
+        with:
+          script: |
+            const opamPath = await io.which('opam', true);
+            console.log('opam executable found: %s', opamPath);
+
+            const fs = require('fs/promises');
+            await fs.writeFile('.opam-path', opamPath, 'utf-8');
+            console.log('stored path to .opam-path');
+
       - name: Install OPAM dependencies (incl. bisect_ppx)
+        if: steps.cache-opam-env.outputs.cache-hit != 'true'
         run: opam install . --deps-only --with-test --with-dev-setup
 
+      # Only save on master pushes. PR-scoped caches are tied to
+      # refs/pull/<n>/merge and can't be restored by master or other PRs,
+      # so they'd just consume storage and evict the shared entry. PRs
+      # still get full restore from the latest master-saved cache.
+      - name: Save OPAM environment
+        if: steps.cache-opam-env.outputs.cache-hit != 'true' && github.event_name == 'push' && github.ref == 'refs/heads/master'
+        uses: actions/cache/save@v5
+        with:
+          path: |
+            ${{ runner.tool_cache }}/opam
+            ~/.opam
+            _opam
+            .opam-path
+          key: ${{ env.opam_cache_key }}
+
+      - name: Use cached OPAM environment
+        if: steps.cache-opam-env.outputs.cache-hit == 'true'
+        run: |
+          # https://github.com/ocaml/setup-ocaml/blob/v3.6.0/packages/setup-ocaml/src/installer.ts
+          echo "OPAMCOLOR=always"                    >> "$GITHUB_ENV"
+          echo "OPAMCONFIRMLEVEL=unsafe-yes"         >> "$GITHUB_ENV"
+          echo "OPAMDOWNLOADJOBS=4"                  >> "$GITHUB_ENV"
+          echo "OPAMERRLOGLEN=0"                     >> "$GITHUB_ENV"
+          echo "OPAMEXTERNALSOLVER=builtin-0install" >> "$GITHUB_ENV"
+          echo "OPAMPRECISETRACKING=1"               >> "$GITHUB_ENV"
+          echo "OPAMRETRIES=10"                      >> "$GITHUB_ENV"
+          echo "OPAMSOLVERTIMEOUT=600"               >> "$GITHUB_ENV"
+          echo "OPAMYES=1"                           >> "$GITHUB_ENV"
+          echo "CLICOLOR_FORCE=1"                    >> "$GITHUB_ENV"
+          echo "OPAMROOT=$HOME/.opam"                >> "$GITHUB_ENV"
+
+          OPAM_PATH="$(cat .opam-path)"
+          chmod +x "$OPAM_PATH"
+          dirname "$OPAM_PATH" >> "$GITHUB_PATH"
+
+      # --- Coverage build cache --------------------------------------------
+      # The bisect_ppx-instrumented dune build is a separate artifact from
+      # the main CI build, so it gets its own key. Only restore/save on
+      # master pushes — PRs share the master cache to stay fast but won't
+      # poison it.
+      - name: Coverage build state key
+        id: coverage-build-state-key
+        run: echo "value=coverage-build-state-v1-${{ env.OS }}-${{ env.SETUP_OCAML_VERSION }}-${{ env.OCAML_COMPILER }}-${{ hashFiles('*.opam', 'compiler/**', 'dune-project') }}" >> $GITHUB_OUTPUT
+
+      - name: Restore coverage build state
+        if: github.base_ref == 'master' || github.ref == 'refs/heads/master'
+        id: coverage-build-state
+        uses: actions/cache/restore@v5
+        with:
+          path: |
+            ~/.cache/dune
+            _build
+          key: ${{ steps.coverage-build-state-key.outputs.value }}
+
       - name: Run coverage
         run: opam exec -- make coverage
 
+      - name: Delete stale coverage build state
+        if: github.event_name == 'push' && github.ref == 'refs/heads/master'
+        run: |
+          gh extension install actions/gh-actions-cache
+          gh actions-cache delete ${{ steps.coverage-build-state-key.outputs.value }} \
+            -R ${{ github.repository }} \
+            -B "$GITHUB_REF" \
+            --confirm || echo "not exist"
+        env:
+          GH_TOKEN: ${{ github.token }}
+
+      - name: Save coverage build state
+        if: github.event_name == 'push' && github.ref == 'refs/heads/master'
+        uses: actions/cache/save@v5
+        with:
+          path: |
+            ~/.cache/dune
+            _build
+          key: ${{ steps.coverage-build-state-key.outputs.value }}
+
       - name: Upload coverage to Codecov
         uses: codecov/codecov-action@v5
         with:
diff --git a/CHANGELOG.md b/CHANGELOG.md
index 85350be7e0..4864ff8e9a 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -47,6 +47,7 @@
 - Expand `super_errors` fixture coverage for warnings and errors. https://github.com/rescript-lang/rescript/pull/8429
 - Run `super_errors` fixtures in parallel (~2.4× faster locally). https://github.com/rescript-lang/rescript/pull/8430
 - Expand `super_errors` fixture coverage for the remaining reachable single-file error variants. https://github.com/rescript-lang/rescript/pull/8432
+- Cache OPAM env, rewatch build, and instrumented dune state in the coverage workflow. https://github.com/rescript-lang/rescript/pull/8434
 
 
 # 13.0.0-alpha.4