diff --git a/database.rules.json b/database.rules.json
index d9dd5e1..458d837 100644
--- a/database.rules.json
+++ b/database.rules.json
@@ -15,6 +15,14 @@
         ".write": false
       }
     },
+    "userVotesByDate": {
+      ".read": false,
+      ".write": false
+    },
+    "userVotesByDateMeta": {
+      ".read": false,
+      ".write": false
+    },
     "cache": {
       ".read": false,
       ".write": false
diff --git a/package.json b/package.json
index 8f48979..2c64d68 100644
--- a/package.json
+++ b/package.json
@@ -14,7 +14,8 @@
     "tools:sheet": "npx tsx scripts/chat-tools-sheet.ts",
     "seed:rewards": "npx tsx scripts/seed-reward-products.ts",
     "upload:reward-assets": "npx tsx scripts/upload-reward-product-assets.ts",
-    "optimize:reward-images": "npx tsx scripts/optimize-reward-product-images.ts"
+    "optimize:reward-images": "npx tsx scripts/optimize-reward-product-images.ts",
+    "backfill:vote-index": "npx tsx scripts/backfill-user-votes-by-date.ts"
   },
   "engines": {
     "node": "24"
diff --git a/scripts/backfill-user-votes-by-date.ts b/scripts/backfill-user-votes-by-date.ts
new file mode 100644
index 0000000..4637845
--- /dev/null
+++ b/scripts/backfill-user-votes-by-date.ts
@@ -0,0 +1,85 @@
+/**
+ * `/userVotes/{uid}/{date}/{gameId}` → `/userVotesByDate/{date}/{uid}/{gameId}` 백필.
+ *
+ * `dailyArchive`는 날짜별 인덱스만 읽는다(원래는 하루치를 위해 `/userVotes` 트리
+ * 전체를 내려받았다). 인덱스 도입 이전에 기록된 투표는 이 스크립트로 옮겨야
+ * 잡의 레거시 폴백 경로를 타지 않는다.
+ *
+ * 멱등하다 — 여러 번 돌려도 같은 값으로 덮어쓸 뿐이다.
+ *
+ * 실행:
+ *   npx tsx scripts/backfill-user-votes-by-date.ts          # 미리보기
+ *   npx tsx scripts/backfill-user-votes-by-date.ts --apply  # 실제 쓰기
+ */
+import "./_bootstrap";
+import { rtdb } from "../src/firebase";
+
+type VoteEntry = { team: string; result?: boolean; cancelled?: boolean };
+type ByUid = Record<string, Record<string, Record<string, VoteEntry>>>;
+
+/** RTDB update는 한 번에 너무 많은 경로를 담으면 실패하므로 나눠 커밋한다. */
+const CHUNK = 500;
+
+async function main(): Promise<void> {
+  const apply = process.argv.includes("--apply");
+
+  const snap = await rtdb.ref("/userVotes").get();
+  if (!snap.exists()) {
+    console.log("no /userVotes data — nothing to backfill");
+    if (apply) await markBackfilled();
+    return;
+  }
+
+  const byUid = snap.val() as ByUid;
+  const updates: Record<string, unknown> = {};
+  let uidCount = 0;
+  let entryCount = 0;
+  const dates = new Set<string>();
+
+  for (const uid of Object.keys(byUid)) {
+    const byDate = byUid[uid] ?? {};
+    let touched = false;
+    for (const date of Object.keys(byDate)) {
+      const games = byDate[date] ?? {};
+      for (const gameId of Object.keys(games)) {
+        updates[`/userVotesByDate/${date}/${uid}/${gameId}`] = games[gameId];
+        entryCount += 1;
+        dates.add(date);
+        touched = true;
+      }
+    }
+    if (touched) uidCount += 1;
+  }
+
+  const paths = Object.keys(updates);
+  console.log(
+    `${entryCount} entries / ${uidCount} uids / ${dates.size} dates` +
+      (apply ? "" : " (dry run — pass --apply to write)")
+  );
+  if (!apply) return;
+
+  for (let i = 0; i < paths.length; i += CHUNK) {
+    const slice: Record<string, unknown> = {};
+    for (const p of paths.slice(i, i + CHUNK)) slice[p] = updates[p];
+    await rtdb.ref().update(slice);
+    console.log(`  written ${Math.min(i + CHUNK, paths.length)}/${paths.length}`);
+  }
+  await markBackfilled();
+  console.log("backfill complete");
+}
+
+/**
+ * 완료 표시. 이 값이 있으면 `dailyArchive`는 인덱스가 비어 있어도
+ * "그날 투표가 없었다"로 해석하고 레거시 전체 스캔 폴백을 쓰지 않는다.
+ */
+async function markBackfilled(): Promise<void> {
+  await rtdb.ref("/userVotesByDateMeta/backfilledAt").set(new Date().toISOString());
+  console.log("marked /userVotesByDateMeta/backfilledAt");
+}
+
+main()
+  .then(() => process.exit(0))
+  .catch((err) => {
+    console.error(err);
+    process.exit(1);
+  });
diff --git a/src/repositories/voteRepository.ts b/src/repositories/voteRepository.ts
index db3e1c3..5f62438 100644
--- a/src/repositories/voteRepository.ts
+++ b/src/repositories/voteRepository.ts
@@ -66,6 +66,7 @@ export async function submitVote(params: {
   updates[`/votes/${gameId}/counts/${side}Count`] = ServerValue.increment(1);
   updates[`/votes/${gameId}/users/${uid}`] = { team };
   updates[`/userVotes/${uid}/${date}/${gameId}`] = { team };
+  updates[byDatePath(uid, date, gameId)] = { team };
   await rtdb.ref().update(updates);
 }
 
@@ -93,9 +94,47 @@ export async function changeVote(params: {
   updates[`/votes/${gameId}/counts/${newSide}Count`] = ServerValue.increment(1);
   updates[`/votes/${gameId}/users/${uid}`] = { team: newTeam };
   updates[`/userVotes/${uid}/${date}/${gameId}`] = { team: newTeam };
+  updates[byDatePath(uid, date, gameId)] = { team: newTeam };
   await rtdb.ref().update(updates);
 }
 
+/**
+ * 날짜별 투표 인덱스 경로 — `/userVotes/{uid}/{date}`의 (date, uid) 전치 미러.
+ *
+ * RTDB에는 부분 노드 읽기가 없어서 `dailyArchive`가 하루치를 처리하려면
+ * `/userVotes` 트리 **전체**(전 유저 × 전 보존 날짜)를 내려받아야 했다.
+ * 쓰기 시점에 fan-out 해 두면 잡이 `/userVotesByDate/{date}` 한 노드만 읽는다.
+ *
+ * ⚠️ 이 경로는 서버 전용이다(교차 유저 데이터). RTDB 규칙에서 read/write 모두 false.
+ */
+function byDatePath(uid: string, date: DateString, gameId: string): string {
+  return `/userVotesByDate/${date}/${uid}/${gameId}`;
+}
+
+/**
+ * 특정 날짜에 투표한 전 유저의 기록을 조회한다(아카이브 전용).
+ *
+ * @returns `{ [uid]: { [gameId]: VoteEntry } }`. 없으면 빈 객체.
+ */
+export async function getVotesByDate(
+  date: DateString
+): Promise<Record<string, Record<string, VoteEntry>>> {
+  const snap = await rtdb.ref(`/userVotesByDate/${date}`).get();
+  return snap.exists() ? (snap.val() as Record<string, Record<string, VoteEntry>>) : {};
+}
+
+/**
+ * 백필 완료 표시를 조회한다.
+ *
+ * 표시가 있으면 날짜별 인덱스가 전 이력을 담고 있다는 뜻이므로, 어떤 날짜가
+ * 비어 있어도 그것은 "그날 투표가 없었다"는 사실이지 인덱스 누락이 아니다.
+ * `dailyArchive`가 레거시 전체 스캔 폴백을 건너뛰는 근거로 쓴다.
+ */
+export async function isVoteDateIndexBackfilled(): Promise<boolean> {
+  const snap = await rtdb.ref("/userVotesByDateMeta/backfilledAt").get();
+  return snap.exists();
+}
+
 /**
  * 특정 유저가 특정 날짜에 한 모든 경기 투표를 조회한다.
  *
@@ -127,7 +166,13 @@ export async function deleteGameVotes(gameId: string): Promise<void> {
  * 탈퇴 비활성화 시 아카이브가 비활성 계정을 판정하지 않도록 정리한다.
  */
 export async function deleteUserVoteIndex(uid: string): Promise<void> {
-  await rtdb.ref(`/userVotes/${uid}`).remove();
+  // 날짜별 미러도 함께 지운다. 어느 날짜에 기록이 있는지는 원본에만 있으므로
+  // 먼저 읽어서 해당 날짜들만 정리한다(유저 1명분이라 작다).
+  const snap = await rtdb.ref(`/userVotes/${uid}`).get();
+  const dates = snap.exists() ? Object.keys(snap.val() as Record<string, unknown>) : [];
+  const updates: Record<string, unknown> = { [`/userVotes/${uid}`]: null };
+  for (const date of dates) updates[`/userVotesByDate/${date}/${uid}`] = null;
+  await rtdb.ref().update(updates);
 }
 
 /**
@@ -139,5 +184,8 @@ export async function deleteUserVoteGame(
   date: DateString,
   gameId: string
 ): Promise<void> {
-  await rtdb.ref(`/userVotes/${uid}/${date}/${gameId}`).remove();
+  await rtdb.ref().update({
+    [`/userVotes/${uid}/${date}/${gameId}`]: null,
+    [byDatePath(uid, date, gameId)]: null,
+  });
 }
diff --git a/src/services/gameResultService.ts b/src/services/gameResultService.ts
index c8b4dd8..aa3a59b 100644
--- a/src/services/gameResultService.ts
+++ b/src/services/gameResultService.ts
@@ -2,7 +2,7 @@ import { FieldValue } from "firebase-admin/firestore";
 import { logger } from "firebase-functions";
 import { firestore, rtdb } from "../firebase";
 import { HttpError } from "../middleware/errors";
-import { getGame } from "../repositories/gameRepository";
+import { getGame, invalidateGameDay } from "../repositories/gameRepository";
 import { getAllUserVotes, deleteGameVotes } from "../repositories/voteRepository";
 import { invalidateStats } from "./statsService";
 import { fromTimestamp } from "../types/dateString";
@@ -36,6 +36,9 @@ export async function processGameEndWithGame(
       : voted === game.winningTeamCode;
     rtdbUpdates[`/userVotes/${uid}/${date}/${gameId}/team`] = voted;
     rtdbUpdates[`/userVotes/${uid}/${date}/${gameId}/result`] = result;
+    // 날짜별 미러도 같은 원자 update에 포함 — 아카이브가 이쪽을 읽는다.
+    rtdbUpdates[`/userVotesByDate/${date}/${uid}/${gameId}/team`] = voted;
+    rtdbUpdates[`/userVotesByDate/${date}/${uid}/${gameId}/result`] = result;
   }
   if (Object.keys(rtdbUpdates).length > 0) {
     await rtdb.ref().update(rtdbUpdates);
@@ -71,5 +74,8 @@ export async function markGameEnded(
     winningTeamCode: isDraw ? FieldValue.delete() : winningTeamCode,
     endedAt: FieldValue.serverTimestamp(),
   });
+  // 스코어·상태가 바뀌었으므로 해당 날짜의 games 캐시를 즉시 버린다.
+  const time = (snap.data() as { time?: { toDate(): Date } }).time;
+  if (time) invalidateGameDay(fromTimestamp(time));
   return { ok: true, gameId };
 }
diff --git a/tests/repositories/voteRepository.test.ts b/tests/repositories/voteRepository.test.ts
index 313914a..ab9409f 100644
--- a/tests/repositories/voteRepository.test.ts
+++ b/tests/repositories/voteRepository.test.ts
@@ -7,6 +7,10 @@ import {
   getCounts,
   getUserDateVotes,
   getUserVote,
+  getVotesByDate,
+  isVoteDateIndexBackfilled,
+  deleteUserVoteGame,
+  deleteUserVoteIndex,
   submitVote,
 } from "../../src/repositories/voteRepository";
 import type { DateString } from "../../src/types/dateString";
@@ -175,3 +179,87 @@ describe("voteRepository (RTDB)", () => {
     });
   });
 });
+
+/**
+ * 날짜별 인덱스는 `/userVotes`의 (date, uid) 전치 미러다. dailyArchive가 이쪽만
+ * 읽으므로, 원본을 바꾸는 모든 경로가 미러도 같이 갱신해야 투표가 유실되지 않는다.
+ */
+describe("voteRepository — /userVotesByDate 미러", () => {
+  beforeEach(async () => {
+    await rtdb.ref("/votes").remove();
+    await rtdb.ref("/userVotes").remove();
+    await rtdb.ref("/userVotesByDate").remove();
+  });
+
+  it("submitVote가 날짜별 인덱스에도 기록한다", async () => {
+    await submitVote({ gameId, uid, date, side: "home", team: "LG" });
+
+    const byDate = await getVotesByDate(date);
+    expect(byDate[uid]?.[gameId]).toEqual({ team: "LG" });
+  });
+
+  it("여러 유저의 같은 날짜 투표가 한 노드에 모인다", async () => {
+    await submitVote({ gameId, uid, date, side: "home", team: "LG" });
+    await submitVote({ gameId, uid: uid2, date, side: "away", team: "KIA" });
+    await submitVote({ gameId: gameId2, uid, date, side: "draw", team: "DRAW" });
+
+    const byDate = await getVotesByDate(date);
+    expect(Object.keys(byDate).sort()).toEqual([uid, uid2].sort());
+    expect(Object.keys(byDate[uid])).toHaveLength(2);
+  });
+
+  it("changeVote가 미러의 팀도 함께 바꾼다", async () => {
+    await submitVote({ gameId, uid, date, side: "home", team: "LG" });
+    await changeVote({
+      gameId, uid, date, oldSide: "home", newSide: "away", newTeam: "KIA",
+    });
+
+    const byDate = await getVotesByDate(date);
+    expect(byDate[uid][gameId]).toEqual({ team: "KIA" });
+  });
+
+  it("deleteUserVoteGame이 양쪽에서 제거한다", async () => {
+    await submitVote({ gameId, uid, date, side: "home", team: "LG" });
+    await submitVote({ gameId: gameId2, uid, date, side: "away", team: "KIA" });
+
+    await deleteUserVoteGame(uid, date, gameId);
+
+    expect(await getUserDateVotes(uid, date)).toEqual({ [gameId2]: { team: "KIA" } });
+    const byDate = await getVotesByDate(date);
+    expect(byDate[uid]).toEqual({ [gameId2]: { team: "KIA" } });
+  });
+
+  it("deleteUserVoteIndex가 그 유저의 모든 날짜 미러를 지운다", async () => {
+    const other = "2026-04-13" as DateString;
+    await submitVote({ gameId, uid, date, side: "home", team: "LG" });
+    await submitVote({ gameId: gameId2, uid, date: other, side: "away", team: "KIA" });
+    await submitVote({ gameId, uid: uid2, date, side: "home", team: "LG" });
+
+    await deleteUserVoteIndex(uid);
+
+    expect(await getUserDateVotes(uid, date)).toEqual({});
+    expect((await getVotesByDate(date))[uid]).toBeUndefined();
+    expect((await getVotesByDate(other))[uid]).toBeUndefined();
+    // 다른 유저 기록은 남아 있어야 한다
+    expect((await getVotesByDate(date))[uid2]).toEqual({ [gameId]: { team: "LG" } });
+  });
+
+  it("투표가 없는 날짜는 빈 객체를 돌려준다", async () => {
+    expect(await getVotesByDate("2026-01-01" as DateString)).toEqual({});
+  });
+});
+
+describe("voteRepository — 백필 완료 표시", () => {
+  beforeEach(async () => {
+    await rtdb.ref("/userVotesByDateMeta").remove();
+  });
+
+  it("표시가 없으면 false", async () => {
+    expect(await isVoteDateIndexBackfilled()).toBe(false);
+  });
+
+  it("표시가 있으면 true — dailyArchive가 레거시 폴백을 건너뛰는 근거", async () => {
+    await rtdb.ref("/userVotesByDateMeta/backfilledAt").set(new Date().toISOString());
+    expect(await isVoteDateIndexBackfilled()).toBe(true);
+  });
+});