diff --git a/backend/routes/upload.js b/backend/routes/upload.js
index e23ec66..5909153 100644
--- a/backend/routes/upload.js
+++ b/backend/routes/upload.js
@@ -4,6 +4,7 @@ const { authenticateToken } = require("../middleware/auth");
 const { uploadPresignLimiter } = require("../middleware/rateLimiter");
 const s3Service = require("../services/s3Service");
 const S3OwnershipService = require("../services/s3OwnershipService");
+const { v4: uuidv4 } = require("uuid");
 const logger = require("../utils/logger");
 const MAX_BATCH_SIZE = 20;
 
@@ -63,6 +64,7 @@ router.post(
 /**
  * POST /api/upload/presign-batch
  * Get presigned URLs for uploading multiple files to S3
+ * All files in a batch share the same UUID base for coordinated variant uploads
  */
 router.post(
   "/presign-batch",
@@ -96,13 +98,17 @@ router.post(
         }
       }
 
+      // Generate one shared UUID for all files in this batch
+      const sharedBaseKey = uuidv4();
+
       const results = await Promise.all(
         files.map((f) =>
           s3Service.getPresignedUploadUrl(
             uploadType,
             f.contentType,
             f.fileName,
-            f.fileSize
+            f.fileSize,
+            sharedBaseKey
           )
         )
       );
@@ -111,9 +117,10 @@ router.post(
         userId: req.user.id,
         uploadType,
         count: results.length,
+        baseKey: sharedBaseKey,
       });
 
-      res.json({ uploads: results });
+      res.json({ uploads: results, baseKey: sharedBaseKey });
     } catch (error) {
       if (error.message.includes("Invalid")) {
         return res.status(400).json({ error: error.message });
diff --git a/backend/services/s3Service.js b/backend/services/s3Service.js
index e4d8d69..f99837d 100644
--- a/backend/services/s3Service.js
+++ b/backend/services/s3Service.js
@@ -112,9 +112,10 @@ class S3Service {
    * @param {string} contentType - MIME type of the file
    * @param {string} fileName - Original filename (used for extension)
    * @param {number} fileSize - File size in bytes (required for size enforcement)
+   * @param {string} [baseKey] - Optional base key (UUID) for coordinated variant uploads
    * @returns {Promise<{uploadUrl: string, key: string, publicUrl: string, expiresAt: Date}>}
    */
-  async getPresignedUploadUrl(uploadType, contentType, fileName, fileSize) {
+  async getPresignedUploadUrl(uploadType, contentType, fileName, fileSize, baseKey = null) {
     if (!this.enabled) {
       throw new Error("S3 storage is not enabled");
     }
@@ -135,8 +136,21 @@ class S3Service {
       );
     }
 
+    // Extract known variant suffix from fileName if present (e.g., "photo_th.jpg" -> "_th")
     const ext = path.extname(fileName) || this.getExtFromMime(contentType);
-    const key = `${config.folder}/${uuidv4()}${ext}`;
+    const baseName = path.basename(fileName, ext);
+
+    // Only recognize known variant suffixes
+    let suffix = "";
+    if (baseName.endsWith("_th")) {
+      suffix = "_th";
+    } else if (baseName.endsWith("_md")) {
+      suffix = "_md";
+    }
+
+    // Use provided baseKey or generate new UUID
+    const uuid = baseKey || uuidv4();
+    const key = `${config.folder}/${uuid}${suffix}${ext}`;
 
     const cacheDirective = config.public ? "public" : "private";
     const command = new PutObjectCommand({
diff --git a/frontend/src/services/uploadService.ts b/frontend/src/services/uploadService.ts
index 1d13458..00b7cfc 100644
--- a/frontend/src/services/uploadService.ts
+++ b/frontend/src/services/uploadService.ts
@@ -59,13 +59,19 @@ export async function getPresignedUrl(
   return response.data;
 }
 
+interface BatchPresignResponse {
+  uploads: PresignedUrlResponse[];
+  baseKey: string;
+}
+
 /**
  * Get presigned URLs for uploading multiple files
+ * All files share the same base UUID for coordinated variant uploads
  */
 export async function getPresignedUrls(
   uploadType: UploadType,
   files: File[]
-): Promise<PresignedUrlResponse[]> {
+): Promise<BatchPresignResponse> {
   const response = await api.post("/upload/presign-batch", {
     uploadType,
     files: files.map((f) => ({
@@ -74,7 +80,7 @@ export async function getPresignedUrls(
       fileSize: f.size,
     })),
   });
-  return response.data.uploads;
+  return { uploads: response.data.uploads, baseKey: response.data.baseKey };
 }
 
 /**
@@ -223,9 +229,9 @@ export async function uploadImageWithVariants(
     throw new Error("Failed to resize image");
   }
 
-  // Get presigned URLs for all variants
+  // Get presigned URLs for all variants (all share same base UUID)
   const files = resizedImages.map((r) => r.file);
-  const presignedUrls = await getPresignedUrls(uploadType, files);
+  const { uploads: presignedUrls } = await getPresignedUrls(uploadType, files);
 
   // Upload all variants in parallel with combined progress
   const totalBytes = files.reduce((sum, f) => sum + f.size, 0);
@@ -251,15 +257,15 @@ export async function uploadImageWithVariants(
   const keys = presignedUrls.map((p) => p.key);
   await confirmUploads(keys);
 
-  // Find the original variant key (no suffix) for database storage
-  const originalIdx = resizedImages.findIndex((r) => r.variant.size === "original");
-  const baseKey = presignedUrls[originalIdx]?.key || presignedUrls[0].key;
-
   if (onProgress) onProgress(100);
 
+  // Use the baseKey returned by the backend (shared UUID for all variants)
+  // The stored key format is: items/uuid.jpg (original), and variants are items/uuid_th.jpg, items/uuid_md.jpg
+  const originalKey = keys.find((k) => !k.includes("_th") && !k.includes("_md")) || keys[0];
+
   return {
-    baseKey,
-    publicUrl: getPublicImageUrl(baseKey),
+    baseKey: originalKey,
+    publicUrl: getPublicImageUrl(originalKey),
     variants: keys,
   };
 }