diff --git a/challenges/2021/21.js b/challenges/2021/21.js
new file mode 100644
index 0000000..d8d8d6b
--- /dev/null
+++ b/challenges/2021/21.js
@@ -0,0 +1,63 @@
+// Part 1
+const solvePuzzle1 = (input) => {
+  let [pos1, pos2] = parseInput(input);
+
+  // die, dieCount, pos1, pos2, score1, score2
+  const state = [0, 0, pos1, pos2, 0, 0];
+
+  const roll = (player, turns = 3) => {
+    while (turns--) {
+      state[0] += 1;
+      state[1] += 1;
+      if (state[0] > 100) state[0] = 1;
+      state[player + 1] += state[0];
+      while (state[player + 1] > 10) state[player + 1] -= 10;
+    }
+  };
+
+  while (true) {
+    roll(1);
+    state[4] += state[2];
+    if (state[4] >= 1000) return state[1] * state[5];
+
+    roll(2);
+    state[5] += state[3];
+    if (state[5] >= 1000) return state[1] * state[4];
+  }
+};
+
+// Part 2
+const solvePuzzle2 = (input) => {
+  const [pos1, pos2] = parseInput(input);
+  return Math.max(...play(pos1, 0, pos2, 0, new Map()));
+};
+
+const play = (pos1, score1, pos2, score2, memo) => {
+  const memoizedResult = memo.get(`${pos1},${score1},${pos2},${score2}`);
+
+  if (memoizedResult) return memoizedResult;
+  if (score1 >= 21) return [1, 0];
+  if (score2 >= 21) return [0, 1];
+
+  let [p1Wins, p2Wins] = [0, 0];
+
+  for (let d1 of [1, 2, 3]) {
+    for (let d2 of [1, 2, 3]) {
+      for (let d3 of [1, 2, 3]) {
+        let pos1Next = pos1 + d1 + d2 + d3;
+        while (pos1Next > 10) pos1Next -= 10;
+
+        const [p2WinsNext, p1WinsNext] = play(pos2, score2, pos1Next, score1 + pos1Next, memo);
+        p1Wins += p1WinsNext;
+        p2Wins += p2WinsNext;
+      }
+    }
+  }
+  memo.set(`${pos1},${score1},${pos2},${score2}`, [p1Wins, p2Wins]);
+  return [p1Wins, p2Wins];
+};
+
+const parseInput = (input) => input.split("\n").map((n) => Number(n[n.length - 1]));
+
+require(__dirname + "/../../utils/test.js").test(__filename, __dirname, solvePuzzle1);
+require(__dirname + "/../../utils/test.js").test(__filename, __dirname, solvePuzzle2);
diff --git a/challenges/2021/inputs/21.txt b/challenges/2021/inputs/21.txt
new file mode 100644
index 0000000..6725b3f
--- /dev/null
+++ b/challenges/2021/inputs/21.txt
@@ -0,0 +1,2 @@
+Player 1 starting position: 8
+Player 2 starting position: 6
\ No newline at end of file