diff --git a/packages/backend/src/core/DriveService.ts b/packages/backend/src/core/DriveService.ts
index 82c447baaa..73125f36d7 100644
--- a/packages/backend/src/core/DriveService.ts
+++ b/packages/backend/src/core/DriveService.ts
@@ -159,6 +159,14 @@ export class DriveService {
 		// thunbnail, webpublic を必要なら生成
 		const alts = await this.generateAlts(path, type, !file.uri);
 
+		if (type && type.startsWith('video/')) {
+			try {
+				await this.videoProcessingService.webOptimizeVideo(path, type);
+			} catch (err) {
+				this.registerLogger.warn(`Video optimization failed: ${err instanceof Error ? err.message : String(err)}`, { error: err });
+			}
+		}
+
 		if (this.meta.useObjectStorage) {
 		//#region ObjectStorage params
 			let [ext] = (name.match(/\.([a-zA-Z0-9_-]+)$/) ?? ['']);
diff --git a/packages/backend/src/core/VideoProcessingService.ts b/packages/backend/src/core/VideoProcessingService.ts
index 747fe4fc7e..3e4fd6a4b0 100644
--- a/packages/backend/src/core/VideoProcessingService.ts
+++ b/packages/backend/src/core/VideoProcessingService.ts
@@ -3,24 +3,41 @@
  * SPDX-License-Identifier: AGPL-3.0-only
  */
 
+import fs from 'node:fs/promises';
 import { Inject, Injectable } from '@nestjs/common';
 import FFmpeg from 'fluent-ffmpeg';
 import { DI } from '@/di-symbols.js';
 import type { Config } from '@/config.js';
 import { ImageProcessingService } from '@/core/ImageProcessingService.js';
 import type { IImage } from '@/core/ImageProcessingService.js';
-import { createTempDir } from '@/misc/create-temp.js';
+import { createTemp, createTempDir } from '@/misc/create-temp.js';
 import { bindThis } from '@/decorators.js';
 import { appendQuery, query } from '@/misc/prelude/url.js';
+import { LoggerService } from '@/core/LoggerService.js';
+import type Logger from '@/logger.js';
+
+// faststart is only supported for MP4, M4A, M4W and MOV files (the MOV family).
+// WebM (and Matroska) files always support faststart-like behavior.
+const supportedMimeTypes = new Map([
+	['video/mp4', 'mp4'],
+	['video/m4a', 'mp4'],
+	['video/m4v', 'mp4'],
+	['video/quicktime', 'mov'],
+]);
 
 @Injectable()
 export class VideoProcessingService {
+	private readonly logger: Logger;
+
 	constructor(
 		@Inject(DI.config)
 		private config: Config,
 
 		private imageProcessingService: ImageProcessingService,
+
+		private loggerService: LoggerService,
 	) {
+		this.logger = this.loggerService.getLogger('video-processing');
 	}
 
 	@bindThis
@@ -60,5 +77,50 @@ export class VideoProcessingService {
 			}),
 		);
 	}
+
+	/**
+	 * Optimize video for web playback by adding faststart flag.
+	 * This allows the video to start playing before it is fully downloaded.
+	 * The original file is modified in-place.
+	 * @param source Path to the video file
+	 * @param mimeType The MIME type of the video
+	 * @returns Promise that resolves when optimization is complete
+	 */
+	@bindThis
+	public async webOptimizeVideo(source: string, mimeType: string): Promise<void> {
+		const outputFormat = supportedMimeTypes.get(mimeType);
+		if (!outputFormat) {
+			this.logger.debug(`Skipping web optimization for unsupported MIME type: ${mimeType}`);
+			return;
+		}
+
+		const [tempPath, cleanup] = await createTemp();
+
+		try {
+			await new Promise<void>((resolve, reject) => {
+				FFmpeg(source)
+					.format(outputFormat) // Specify output format
+					.addOutputOptions('-c copy') // Copy streams without re-encoding
+					.addOutputOptions('-movflags +faststart')
+					.on('error', reject)
+					.on('end', async () => {
+						try {
+							// Replace original file with optimized version
+							await fs.copyFile(tempPath, source);
+							this.logger.info(`Web-optimized video: ${source}`);
+							resolve();
+						} catch (copyError) {
+							reject(copyError);
+						}
+					})
+					.save(tempPath);
+			});
+		} catch (error) {
+			this.logger.warn(`Failed to web-optimize video: ${source}`, { error });
+			throw error;
+		} finally {
+			cleanup();
+		}
+	}
 }