diff --git a/server/src/db/migration/0007_migrateMaintenanceWindowMonitorIdToArray.ts b/server/src/db/migration/0007_migrateMaintenanceWindowMonitorIdToArray.ts
new file mode 100644
index 000000000..a60b8786a
--- /dev/null
+++ b/server/src/db/migration/0007_migrateMaintenanceWindowMonitorIdToArray.ts
@@ -0,0 +1,111 @@
+import mongoose from "mongoose";
+import type { AnyBulkWriteOperation, Document } from "mongodb";
+import { logger } from "@/utils/logger.js";
+import MaintenanceWindowModel from "../models/MaintenanceWindow.js";
+
+type GroupResult = {
+	_id: {
+		teamId: mongoose.Types.ObjectId;
+		name: string;
+		start: Date;
+		end: Date;
+		repeat: number;
+		durationUnit: string;
+		duration: number;
+		active: boolean;
+	};
+	docIds: mongoose.Types.ObjectId[];
+	monitorIds: mongoose.Types.ObjectId[];
+};
+
+export async function migrateMaintenanceWindowMonitorIdToArray(): Promise<void> {
+	const SERVICE_NAME = "Migration:MigrateMaintenanceWindowMonitorIdToArray";
+
+	try {
+		logger.info({
+			service: SERVICE_NAME,
+			message: "Starting migration of MaintenanceWindow.monitorId to monitorIds array (eager merge)",
+		});
+
+		const db = mongoose.connection.db;
+		if (!db) {
+			throw new Error("Database connection is not initialized");
+		}
+
+		// Group old-style docs by their scheduling key.
+		// Skip rows that already have a monitorIds array — those are either already migrated
+		// or in a partially-migrated state that needs manual review.
+		const groups: GroupResult[] = await MaintenanceWindowModel.aggregate<GroupResult>([
+			{
+				$match: {
+					monitorId: { $exists: true, $ne: null },
+					monitorIds: { $exists: false },
+				},
+			},
+			{
+				$group: {
+					_id: {
+						teamId: "$teamId",
+						name: "$name",
+						start: "$start",
+						end: "$end",
+						repeat: "$repeat",
+						durationUnit: "$durationUnit",
+						duration: "$duration",
+						active: "$active",
+					},
+					docIds: { $push: "$_id" },
+					monitorIds: { $addToSet: "$monitorId" },
+				},
+			},
+		]);
+
+		if (groups.length === 0) {
+			logger.info({ service: SERVICE_NAME, message: "No MaintenanceWindow documents needed migration" });
+			return;
+		}
+
+		const bulkOps: AnyBulkWriteOperation<Document>[] = [];
+		let originalDocCount = 0;
+
+		for (const group of groups) {
+			const { docIds, monitorIds } = group;
+			originalDocCount += docIds.length;
+			const [survivorId, ...duplicateIds] = docIds;
+
+			bulkOps.push({
+				updateOne: {
+					filter: { _id: survivorId },
+					update: {
+						$set: { monitorIds },
+						$unset: { monitorId: "" },
+					},
+				},
+			});
+
+			if (duplicateIds.length > 0) {
+				bulkOps.push({
+					deleteMany: {
+						filter: { _id: { $in: duplicateIds } },
+					},
+				});
+			}
+		}
+
+		const result = await MaintenanceWindowModel.collection.bulkWrite(bulkOps, { ordered: true });
+		const deletedCount = originalDocCount - groups.length;
+
+		logger.info({
+			service: SERVICE_NAME,
+			message: `Migration complete. Consolidated ${originalDocCount} old document(s) into ${groups.length} group window(s); deleted ${deletedCount} duplicate(s).`,
+		});
+		logger.debug({
+			service: SERVICE_NAME,
+			message: `bulkWrite result: modified=${result.modifiedCount}, deleted=${result.deletedCount}`,
+		});
+	} catch (error) {
+		const errorMessage = error instanceof Error ? error.message : String(error);
+		logger.error({ service: SERVICE_NAME, message: `Error during MaintenanceWindow migration: ${errorMessage}` });
+		throw error;
+	}
+}
diff --git a/server/src/db/migration/index.ts b/server/src/db/migration/index.ts
index 5b74a96fc..61785118e 100644
--- a/server/src/db/migration/index.ts
+++ b/server/src/db/migration/index.ts
@@ -5,6 +5,7 @@ import { fixInfrastructureThresholds } from "./0004_fixInfrastructureThresholds.
 import MigrationModel from "../models/Migration.js";
 import { migrateStatusPageTypeToArray } from "./0005_migrateStatusPageTypeToArray.js";
 import { cleanupDuplicateMonitorStatsForUniqueIndex } from "./0006_cleanupDuplicateMonitorStatsForUniqueIndex.js";
+import { migrateMaintenanceWindowMonitorIdToArray } from "./0007_migrateMaintenanceWindowMonitorIdToArray.js";
 import type { ILogger } from "@/utils/logger.js";
 
 type MigrationEntry = {
@@ -19,6 +20,7 @@ const migrations: MigrationEntry[] = [
 	{ name: "0004_fixInfrastructureThresholds", execute: fixInfrastructureThresholds },
 	{ name: "0005_migrateStatusPageTypeToArray", execute: migrateStatusPageTypeToArray },
 	{ name: "0006_cleanupDuplicateMonitorStatsForUniqueIndex", execute: cleanupDuplicateMonitorStatsForUniqueIndex },
+	{ name: "0007_migrateMaintenanceWindowMonitorIdToArray", execute: migrateMaintenanceWindowMonitorIdToArray },
 ];
 
 const runMigrations = async (logger?: ILogger) => {