diff --git a/.clang-format b/.clang-format
new file mode 100644
index 000000000..44397b42f
--- /dev/null
+++ b/.clang-format
@@ -0,0 +1,4 @@
+BasedOnStyle: Google
+IndentWidth: 4
+AccessModifierOffset: -4
+SortIncludes: false # FIXME: https://github.com/4jcraft/4jcraft/issues/225
\ No newline at end of file
diff --git a/.github/scripts/check-clang-format.sh b/.github/scripts/check-clang-format.sh
new file mode 100644
index 000000000..3efb79fe5
--- /dev/null
+++ b/.github/scripts/check-clang-format.sh
@@ -0,0 +1,34 @@
+#!/usr/bin/env bash
+
+set -euo pipefail
+
+formatter="${CLANG_FORMAT_BIN:-clang-format-19}"
+base_ref="${1:-}"
+head_ref="${2:-HEAD}"
+
+if [[ -z "$base_ref" ]]; then
+    if git rev-parse --verify HEAD^ >/dev/null 2>&1; then
+        base_ref="$(git rev-parse HEAD^)"
+    else
+        echo "No comparison base available; skipping clang-format check."
+        exit 0
+    fi
+fi
+
+diff_output="$(
+    git diff --name-only --diff-filter=ACMR "$base_ref" "$head_ref" -- \
+        '*.c' '*.cc' '*.cpp' '*.cxx' '*.h' '*.hh' '*.hpp' '*.hxx' '*.inl'
+)"
+
+if [[ -z "$diff_output" ]]; then
+    echo "No changed C/C++ files to check."
+    exit 0
+fi
+
+mapfile -t files <<<"$diff_output"
+
+echo "Checking formatting for changed files:"
+printf '  %s\n' "${files[@]}"
+
+"$formatter" --version
+"$formatter" --dry-run --Werror "${files[@]}"
\ No newline at end of file
diff --git a/.github/workflows/clang-format.yml b/.github/workflows/clang-format.yml
new file mode 100644
index 000000000..a125848e6
--- /dev/null
+++ b/.github/workflows/clang-format.yml
@@ -0,0 +1,69 @@
+name: Clang Format
+
+on:
+  push:
+    paths:
+      - '**.cpp'
+      - '**.h'
+      - '**.c'
+      - '**.cc'
+      - '**.cxx'
+      - '**.hh'
+      - '**.hpp'
+      - '**.hxx'
+      - '**.inl'
+      - '.clang-format'
+      - '.github/workflows/clang-format.yml'
+      - '.github/scripts/check-clang-format.sh'
+  pull_request:
+    paths:
+      - '**.cpp'
+      - '**.h'
+      - '**.c'
+      - '**.cc'
+      - '**.cxx'
+      - '**.hh'
+      - '**.hpp'
+      - '**.hxx'
+      - '**.inl'
+      - '.clang-format'
+      - '.github/workflows/clang-format.yml'
+      - '.github/scripts/check-clang-format.sh'
+
+jobs:
+  clang-format:
+    runs-on: ubuntu-24.04
+    concurrency:
+      group: clang-format-${{ github.ref }}
+      cancel-in-progress: true
+    steps:
+      - name: Checkout repository
+        uses: actions/checkout@v4
+        with:
+          fetch-depth: 0
+
+      - name: Install clang-format
+        run: |
+          sudo apt-get update
+          sudo apt-get install -y clang-format-19
+
+      - name: Check changed files
+        env:
+          CLANG_FORMAT_BIN: clang-format-19
+          EVENT_NAME: ${{ github.event_name }}
+          PR_BASE_REF: ${{ github.event.pull_request.base.ref }}
+          PR_BASE_SHA: ${{ github.event.pull_request.base.sha }}
+          BEFORE_SHA: ${{ github.event.before }}
+          CURRENT_SHA: ${{ github.sha }}
+        run: |
+          set -euo pipefail
+
+          BASE_SHA=""
+          if [ "$EVENT_NAME" = "pull_request" ]; then
+            git fetch --no-tags origin "$PR_BASE_REF" --depth=1
+            BASE_SHA="$(git merge-base "origin/$PR_BASE_REF" "$CURRENT_SHA")"
+          elif [ -n "$BEFORE_SHA" ] && [ "$BEFORE_SHA" != "0000000000000000000000000000000000000000" ]; then
+            BASE_SHA="$BEFORE_SHA"
+          fi
+
+          bash ./.github/scripts/check-clang-format.sh "$BASE_SHA" "$CURRENT_SHA"
\ No newline at end of file
diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
index 8f6de058d..7c2dcf001 100644
--- a/CONTRIBUTING.md
+++ b/CONTRIBUTING.md
@@ -32,7 +32,9 @@ Commit names should clearly describe what was changed in the commit. [Convention
 
 ### Keep code clean and readable.
 
-At this time, we do not have a style guide or rules for how code should be formatted. In general, code should be readable and try to match the styling and conventions of whatever is around it.
+Code formatting is defined by the repository's [`.clang-format`](./.clang-format) file. If you are touching C or C++ source, format the files you changed before opening or updating a pull request.
+
+CI checks formatting on changed C and C++ files, so local formatting mismatches will fail the relevant workflow.
 
 ### Avoid changing in-game behavior.