diff --git a/build_wasm.sh b/build_wasm.sh
new file mode 100755
index 0000000..c826544
--- /dev/null
+++ b/build_wasm.sh
@@ -0,0 +1,40 @@
+#!/usr/bin/env bash
+# Rebuild the solitaire_wasm crate and install the output into
+# solitaire_server/web/pkg/ so the server can serve the replay viewer.
+#
+# Prerequisites:
+#   cargo install wasm-pack
+#   rustup target add wasm32-unknown-unknown
+#
+# Run from the repo root:
+#   ./build_wasm.sh
+#
+# The generated files (solitaire_wasm.js + solitaire_wasm_bg.wasm) are
+# committed to git so self-hosters who don't touch the WASM crate can
+# skip this step.  Regenerate after any change to solitaire_wasm/ or
+# solitaire_core/.
+
+set -euo pipefail
+
+REPO_ROOT="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
+OUT_DIR="$REPO_ROOT/solitaire_server/web/pkg"
+
+if ! command -v wasm-pack &> /dev/null; then
+    echo "error: wasm-pack not found." >&2
+    echo "  Install with: cargo install wasm-pack" >&2
+    exit 1
+fi
+
+echo "Building solitaire_wasm (target: web)..."
+wasm-pack build \
+    --target web \
+    --out-dir "$OUT_DIR" \
+    --no-typescript \
+    "$REPO_ROOT/solitaire_wasm"
+
+# wasm-pack writes a package.json and .gitignore into the output dir.
+# Remove them — we manage the output directory ourselves.
+rm -f "$OUT_DIR/package.json" "$OUT_DIR/.gitignore"
+
+echo "Done. Output:"
+ls -lh "$OUT_DIR"