diff --git a/news/changelog-1.9.md b/news/changelog-1.9.md
index 1d2ed43075d..3de5e420900 100644
--- a/news/changelog-1.9.md
+++ b/news/changelog-1.9.md
@@ -135,6 +135,7 @@ All changes included in 1.9:
 
 ### Confluence
 
+- ([#12558](https://github.com/quarto-dev/quarto-cli/issues/12558)): Fix 500 error when updating multiple Confluence attachments. Attachments are now uploaded sequentially instead of concurrently. (author: @fkgruber)
 - ([#13414](https://github.com/quarto-dev/quarto-cli/issues/13414)): Be more forgiving when Confluence server returns malformed JSON response. (author: @m1no)
 
 ### `gh-pages`
diff --git a/src/publish/confluence/confluence.ts b/src/publish/confluence/confluence.ts
index 99233cac9b8..aa043431611 100644
--- a/src/publish/confluence/confluence.ts
+++ b/src/publish/confluence/confluence.ts
@@ -87,6 +87,7 @@ import {
 } from "./confluence-verify.ts";
 import {
   DELETE_DISABLED,
+  ATTACHMENT_UPLOAD_DELAY_MS,
   DELETE_SLEEP_MILLIS,
   DESCENDANT_PAGE_SIZE,
   EXIT_ON_ERROR,
@@ -435,15 +436,25 @@ async function publish(
         LogPrefix.ATTACHMENT,
       );
 
-      const uploadAttachmentsResult = await Promise.all(
-        uploadAttachments(
+      const uploadAttachmentsResult: (AttachmentSummary | null)[] = [];
+
+      for (let i = 0; i < attachmentsToUpload.length; i++) {
+        // Start exactly ONE upload by calling the helper with a single attachment
+        const tasks = uploadAttachments(
           publishFiles.baseDir,
-          attachmentsToUpload,
+          [attachmentsToUpload[i]],  // <-- one at a time
           toUpdate.id,
           fileName,
           existingAttachments,
-        ),
-      );
+        );
+
+        const res = await tasks[0];
+        uploadAttachmentsResult.push(res);
+
+        if (i < attachmentsToUpload.length - 1) {
+          await sleep(ATTACHMENT_UPLOAD_DELAY_MS);
+        }
+      }
       trace(
         "uploadAttachmentsResult",
         uploadAttachmentsResult,
diff --git a/src/publish/confluence/constants.ts b/src/publish/confluence/constants.ts
index 7803856a629..533dff14644 100644
--- a/src/publish/confluence/constants.ts
+++ b/src/publish/confluence/constants.ts
@@ -18,6 +18,9 @@ export const V2EDITOR_METADATA = {
 
 export const DELETE_SLEEP_MILLIS = 1000; //TODO replace with polling
 
+// Empirically determined delay to avoid Confluence 500 errors on concurrent attachment updates
+export const ATTACHMENT_UPLOAD_DELAY_MS = 800;
+
 export const CAN_SET_PERMISSIONS_DISABLED = "CONFLUENCE_LOCAL_STORAGE_CAN_SET_PERMISSIONS_DISABLED";
 
 export const CAN_SET_PERMISSIONS_ENABLED_CACHED = "CONFLUENCE_LOCAL_STORAGE_CAN_SET_PERMISSIONS_ENABLED_CACHED"