diff --git a/setup-r-dependencies/README.md b/setup-r-dependencies/README.md
index 8705132c..e361a3c4 100644
--- a/setup-r-dependencies/README.md
+++ b/setup-r-dependencies/README.md
@@ -301,6 +301,10 @@ On Windows you can usually use `pacman` that is included in Rtools4, or
           choco install mariadb
 ```
 
+`setup-r-dependencies` also sets `JAVA_HOME` to Java 21 on GitHub-hosted
+Windows runners (`JAVA_HOME_21_X64` on x64, `JAVA_HOME_21_AARCH64` on arm64)
+if `JAVA_HOME` is not already set.
+
 # Installing the latest dependencies
 
 Note that `setup-r-dependencies` does _not_ necessarily install the latest
diff --git a/setup-r-dependencies/action.yaml b/setup-r-dependencies/action.yaml
index e7082ba1..bb19e3ef 100644
--- a/setup-r-dependencies/action.yaml
+++ b/setup-r-dependencies/action.yaml
@@ -128,6 +128,20 @@ runs:
           cat("::endgroup::\n")
         shell: Rscript {0}
 
+      - name: Set JAVA_HOME (Windows)
+        if: ${{ runner.os == 'Windows' }}
+        shell: bash
+        run: |
+          if [ -z "${JAVA_HOME:-}" ]; then
+            arch="${RUNNER_ARCH^^}"
+            case "${arch}" in
+              X64) echo "JAVA_HOME=${JAVA_HOME_21_X64}" >> $GITHUB_ENV ;;
+              # GitHub runners expose ARM64 Java 21 in JAVA_HOME_21_AARCH64.
+              ARM64) echo "JAVA_HOME=${JAVA_HOME_21_AARCH64}" >> $GITHUB_ENV ;;
+              *) echo "Warning: unsupported Windows architecture '${arch}', JAVA_HOME not set by setup-r-dependencies." >&2 ;;
+            esac
+          fi
+
       - name: Install pak (Unix)
         if: ${{ runner.os != 'Windows' && inputs.pak-version != 'none' }}
         run: |