diff --git a/packages/react-devtools-shared/src/backend/fiber/renderer.js b/packages/react-devtools-shared/src/backend/fiber/renderer.js
index b927b1a58fd..d6186935fef 100644
--- a/packages/react-devtools-shared/src/backend/fiber/renderer.js
+++ b/packages/react-devtools-shared/src/backend/fiber/renderer.js
@@ -15,6 +15,7 @@ import type {
   ReactIOInfo,
   ReactStackTrace,
   ReactCallSite,
+  Wakeable,
 } from 'shared/ReactTypes';
 
 import type {HooksTree} from 'react-debug-tools/src/ReactDebugHooks';
@@ -87,6 +88,10 @@ import {
   SUSPENSE_TREE_OPERATION_REMOVE,
   SUSPENSE_TREE_OPERATION_REORDER_CHILDREN,
   SUSPENSE_TREE_OPERATION_RESIZE,
+  UNKNOWN_SUSPENDERS_NONE,
+  UNKNOWN_SUSPENDERS_REASON_PRODUCTION,
+  UNKNOWN_SUSPENDERS_REASON_OLD_VERSION,
+  UNKNOWN_SUSPENDERS_REASON_THROWN_PROMISE,
 } from '../../constants';
 import {inspectHooksOfFiber} from 'react-debug-tools';
 import {
@@ -296,6 +301,9 @@ type SuspenseNode = {
   // Track whether any of the items in suspendedBy are unique this this Suspense boundaries or if they're all
   // also in the parent sets. This determine whether this could contribute in the loading sequence.
   hasUniqueSuspenders: boolean,
+  // Track whether anything suspended in this boundary that we can't track either because it was using throw
+  // a promise, an older version of React or because we're inspecting prod.
+  hasUnknownSuspenders: boolean,
 };
 
 function createSuspenseNode(
@@ -309,6 +317,7 @@ function createSuspenseNode(
     rects: null,
     suspendedBy: new Map(),
     hasUniqueSuspenders: false,
+    hasUnknownSuspenders: false,
   });
 }
 
@@ -2745,6 +2754,8 @@ export function attach(
         parentSuspenseNode.hasUniqueSuspenders = true;
       }
     }
+    // We have observed at least one known reason this might have been suspended.
+    parentSuspenseNode.hasUnknownSuspenders = false;
     // Suspending right below the root is not attributed to any particular component in UI
     // other than the SuspenseNode and the HostRoot's FiberInstance.
     const suspendedBy = parentInstance.suspendedBy;
@@ -2783,6 +2794,7 @@ export function attach(
         // It can now be marked as having unique suspenders. We can skip its children
         // since they'll still be blocked by this one.
         node.hasUniqueSuspenders = true;
+        node.hasUnknownSuspenders = false;
       } else if (node.firstChild !== null) {
         node = node.firstChild;
         continue;
@@ -3458,6 +3470,25 @@ export function attach(
     insertSuspendedBy(asyncInfo);
   }
 
+  function trackThrownPromisesFromRetryCache(
+    suspenseNode: SuspenseNode,
+    retryCache: ?WeakSet<Wakeable>,
+  ): void {
+    if (retryCache != null) {
+      // If a Suspense boundary ever committed in fallback state with a retryCache, that
+      // suggests that something unique to that boundary was suspensey since otherwise
+      // it wouldn't have thrown and so never created the retryCache.
+      // Unfortunately if we don't have any DEV time debug info or debug thenables then
+      // we have no meta data to show. However, we still mark this Suspense boundary as
+      // participating in the loading sequence since apparently it can suspend.
+      suspenseNode.hasUniqueSuspenders = true;
+      // We have not seen any reason yet for why this suspense node might have been
+      // suspended but it clearly has been at some point. If we later discover a reason
+      // we'll clear this flag again.
+      suspenseNode.hasUnknownSuspenders = true;
+    }
+  }
+
   function mountVirtualChildrenRecursively(
     firstChild: Fiber,
     lastChild: null | Fiber, // non-inclusive
@@ -3749,6 +3780,9 @@ export function attach(
       } else if (fiber.tag === SuspenseComponent && OffscreenComponent === -1) {
         // Legacy Suspense without the Offscreen wrapper. For the modern Suspense we just handle the
         // Offscreen wrapper itself specially.
+        if (newSuspenseNode !== null) {
+          trackThrownPromisesFromRetryCache(newSuspenseNode, fiber.stateNode);
+        }
         const isTimedOut = fiber.memoizedState !== null;
         if (isTimedOut) {
           // Special case: if Suspense mounts in a timed-out state,
@@ -3791,6 +3825,9 @@ export function attach(
             'There should always be an Offscreen Fiber child in a Suspense boundary.',
           );
         }
+
+        trackThrownPromisesFromRetryCache(newSuspenseNode, fiber.stateNode);
+
         const fallbackFiber = contentFiber.sibling;
 
         // First update only the Offscreen boundary. I.e. the main content.
@@ -4600,6 +4637,18 @@ export function attach(
       const prevWasHidden = isOffscreen && prevFiber.memoizedState !== null;
       const nextIsHidden = isOffscreen && nextFiber.memoizedState !== null;
 
+      if (isLegacySuspense) {
+        if (
+          fiberInstance !== null &&
+          fiberInstance.suspenseNode !== null &&
+          (prevFiber.stateNode === null) !== (nextFiber.stateNode === null)
+        ) {
+          trackThrownPromisesFromRetryCache(
+            fiberInstance.suspenseNode,
+            nextFiber.stateNode,
+          );
+        }
+      }
       // The logic below is inspired by the code paths in updateSuspenseComponent()
       // inside ReactFiberBeginWork in the React source code.
       if (prevDidTimeout && nextDidTimeOut) {
@@ -4726,6 +4775,13 @@ export function attach(
         const prevFallbackFiber = prevContentFiber.sibling;
         const nextFallbackFiber = nextContentFiber.sibling;
 
+        if ((prevFiber.stateNode === null) !== (nextFiber.stateNode === null)) {
+          trackThrownPromisesFromRetryCache(
+            fiberInstance.suspenseNode,
+            nextFiber.stateNode,
+          );
+        }
+
         // First update only the Offscreen boundary. I.e. the main content.
         updateFlags |= updateVirtualChildrenRecursively(
           nextContentFiber,
@@ -6090,6 +6146,23 @@ export function attach(
       getNearestSuspenseNode(fiberInstance),
     );
 
+    let unknownSuspenders = UNKNOWN_SUSPENDERS_NONE;
+    if (
+      fiberInstance.suspenseNode !== null &&
+      fiberInstance.suspenseNode.hasUnknownSuspenders &&
+      !isTimedOutSuspense
+    ) {
+      // Something unknown threw to suspended this boundary. Let's figure out why that might be.
+      if (renderer.bundleType === 0) {
+        unknownSuspenders = UNKNOWN_SUSPENDERS_REASON_PRODUCTION;
+      } else if (!('_debugInfo' in fiber)) {
+        // TODO: We really should detect _debugThenable and the auto-instrumentation for lazy/thenables too.
+        unknownSuspenders = UNKNOWN_SUSPENDERS_REASON_OLD_VERSION;
+      } else {
+        unknownSuspenders = UNKNOWN_SUSPENDERS_REASON_THROWN_PROMISE;
+      }
+    }
+
     return {
       id: fiberInstance.id,
 
@@ -6153,6 +6226,7 @@ export function attach(
 
       suspendedBy: suspendedBy,
       suspendedByRange: suspendedByRange,
+      unknownSuspenders: unknownSuspenders,
 
       // List of owners
       owners,
@@ -6267,6 +6341,7 @@ export function attach(
               serializeAsyncInfo(info, virtualInstance, null),
             ),
       suspendedByRange: suspendedByRange,
+      unknownSuspenders: UNKNOWN_SUSPENDERS_NONE,
 
       // List of owners
       owners,
diff --git a/packages/react-devtools-shared/src/backend/legacy/renderer.js b/packages/react-devtools-shared/src/backend/legacy/renderer.js
index 82a3a8d1905..764f926a2e7 100644
--- a/packages/react-devtools-shared/src/backend/legacy/renderer.js
+++ b/packages/react-devtools-shared/src/backend/legacy/renderer.js
@@ -34,6 +34,7 @@ import {
   TREE_OPERATION_ADD,
   TREE_OPERATION_REMOVE,
   TREE_OPERATION_REORDER_CHILDREN,
+  UNKNOWN_SUSPENDERS_NONE,
 } from '../../constants';
 import {decorateMany, forceUpdate, restoreMany} from './utils';
 
@@ -859,6 +860,7 @@ export function attach(
       // Not supported in legacy renderers.
       suspendedBy: [],
       suspendedByRange: null,
+      unknownSuspenders: UNKNOWN_SUSPENDERS_NONE,
 
       // List of owners
       owners,
diff --git a/packages/react-devtools-shared/src/backend/types.js b/packages/react-devtools-shared/src/backend/types.js
index 0d2a86beb5b..1da09046862 100644
--- a/packages/react-devtools-shared/src/backend/types.js
+++ b/packages/react-devtools-shared/src/backend/types.js
@@ -34,6 +34,7 @@ import type {TimelineDataExport} from 'react-devtools-timeline/src/types';
 import type {BackendBridge} from 'react-devtools-shared/src/bridge';
 import type {ReactFunctionLocation, ReactStackTrace} from 'shared/ReactTypes';
 import type Agent from './agent';
+import type {UnknownSuspendersReason} from '../constants';
 
 type BundleType =
   | 0 // PROD
@@ -301,6 +302,7 @@ export type InspectedElement = {
   // Things that suspended this Instances
   suspendedBy: Object, // DehydratedData or Array<SerializedAsyncInfo>
   suspendedByRange: null | [number, number],
+  unknownSuspenders: UnknownSuspendersReason,
 
   // List of owners
   owners: Array<SerializedElement> | null,
diff --git a/packages/react-devtools-shared/src/backendAPI.js b/packages/react-devtools-shared/src/backendAPI.js
index dcaec7dc034..16b1b8f89f8 100644
--- a/packages/react-devtools-shared/src/backendAPI.js
+++ b/packages/react-devtools-shared/src/backendAPI.js
@@ -271,6 +271,7 @@ export function convertInspectedElementBackendToFrontend(
     warnings,
     suspendedBy,
     suspendedByRange,
+    unknownSuspenders,
     nativeTag,
   } = inspectedElementBackend;
 
@@ -315,6 +316,7 @@ export function convertInspectedElementBackendToFrontend(
         ? []
         : hydratedSuspendedBy.map(backendToFrontendSerializedAsyncInfo),
     suspendedByRange,
+    unknownSuspenders,
     nativeTag,
   };
 
diff --git a/packages/react-devtools-shared/src/constants.js b/packages/react-devtools-shared/src/constants.js
index ce6ed0b308a..8071d3d4a2c 100644
--- a/packages/react-devtools-shared/src/constants.js
+++ b/packages/react-devtools-shared/src/constants.js
@@ -32,6 +32,13 @@ export const SUSPENSE_TREE_OPERATION_RESIZE = 11;
 export const PROFILING_FLAG_BASIC_SUPPORT = 0b01;
 export const PROFILING_FLAG_TIMELINE_SUPPORT = 0b10;
 
+export const UNKNOWN_SUSPENDERS_NONE: UnknownSuspendersReason = 0; // If we had at least one debugInfo, then that might have been the reason.
+export const UNKNOWN_SUSPENDERS_REASON_PRODUCTION: UnknownSuspendersReason = 1; // We're running in prod. That might be why we had unknown suspenders.
+export const UNKNOWN_SUSPENDERS_REASON_OLD_VERSION: UnknownSuspendersReason = 2; // We're running an old version of React that doesn't have full coverage. That might be the reason.
+export const UNKNOWN_SUSPENDERS_REASON_THROWN_PROMISE: UnknownSuspendersReason = 3; // If we're in dev, didn't detect and debug info and still suspended (other than CSS/image) the only reason is thrown promise.
+
+export opaque type UnknownSuspendersReason = 0 | 1 | 2 | 3;
+
 export const LOCAL_STORAGE_DEFAULT_TAB_KEY = 'React::DevTools::defaultTab';
 export const LOCAL_STORAGE_COMPONENT_FILTER_PREFERENCES_KEY =
   'React::DevTools::componentFilters';
diff --git a/packages/react-devtools-shared/src/devtools/views/Components/InspectedElementSharedStyles.css b/packages/react-devtools-shared/src/devtools/views/Components/InspectedElementSharedStyles.css
index 0fb5107361c..978077d2d9a 100644
--- a/packages/react-devtools-shared/src/devtools/views/Components/InspectedElementSharedStyles.css
+++ b/packages/react-devtools-shared/src/devtools/views/Components/InspectedElementSharedStyles.css
@@ -52,6 +52,15 @@
   min-width: 1rem;
 }
 
+.InfoRow {
+  border-top: 1px solid var(--color-border);
+  padding: 0.5rem 1rem;
+}
+
+.InfoRow:last-child {
+  margin-bottom: -0.25rem;
+}
+
 .CollapsableRow {
   border-top: 1px solid var(--color-border);
 }
diff --git a/packages/react-devtools-shared/src/devtools/views/Components/InspectedElementSuspendedBy.js b/packages/react-devtools-shared/src/devtools/views/Components/InspectedElementSuspendedBy.js
index 3527c23b066..451b53b4ac5 100644
--- a/packages/react-devtools-shared/src/devtools/views/Components/InspectedElementSuspendedBy.js
+++ b/packages/react-devtools-shared/src/devtools/views/Components/InspectedElementSuspendedBy.js
@@ -27,6 +27,13 @@ import type {
 } from 'react-devtools-shared/src/frontend/types';
 import type {FrontendBridge} from 'react-devtools-shared/src/bridge';
 
+import {
+  UNKNOWN_SUSPENDERS_NONE,
+  UNKNOWN_SUSPENDERS_REASON_PRODUCTION,
+  UNKNOWN_SUSPENDERS_REASON_OLD_VERSION,
+  UNKNOWN_SUSPENDERS_REASON_THROWN_PROMISE,
+} from '../../../constants';
+
 type RowProps = {
   bridge: FrontendBridge,
   element: Element,
@@ -295,7 +302,10 @@ export default function InspectedElementSuspendedBy({
   const {suspendedBy, suspendedByRange} = inspectedElement;
 
   // Skip the section if nothing suspended this component.
-  if (suspendedBy == null || suspendedBy.length === 0) {
+  if (
+    (suspendedBy == null || suspendedBy.length === 0) &&
+    inspectedElement.unknownSuspenders === UNKNOWN_SUSPENDERS_NONE
+  ) {
     return null;
   }
 
@@ -327,9 +337,41 @@ export default function InspectedElementSuspendedBy({
     minTime = maxTime - 25;
   }
 
-  const sortedSuspendedBy = suspendedBy.slice(0);
+  const sortedSuspendedBy = suspendedBy === null ? [] : suspendedBy.slice(0);
   sortedSuspendedBy.sort(compareTime);
 
+  let unknownSuspenders = null;
+  switch (inspectedElement.unknownSuspenders) {
+    case UNKNOWN_SUSPENDERS_REASON_PRODUCTION:
+      unknownSuspenders = (
+        <div className={styles.InfoRow}>
+          Something suspended but we don't know the exact reason in production
+          builds of React. Test this in development mode to see exactly what
+          might suspend.
+        </div>
+      );
+      break;
+    case UNKNOWN_SUSPENDERS_REASON_OLD_VERSION:
+      unknownSuspenders = (
+        <div className={styles.InfoRow}>
+          Something suspended but we don't track all the necessary information
+          in older versions of React. Upgrade to the latest version of React to
+          see exactly what might suspend.
+        </div>
+      );
+      break;
+    case UNKNOWN_SUSPENDERS_REASON_THROWN_PROMISE:
+      unknownSuspenders = (
+        <div className={styles.InfoRow}>
+          Something threw a Promise to suspend this boundary. It's likely an
+          outdated version of a library that doesn't yet fully take advantage of
+          use(). Upgrade your data fetching library to see exactly what might
+          suspend.
+        </div>
+      );
+      break;
+  }
+
   return (
     <div>
       <div className={styles.HeaderRow}>
@@ -351,6 +393,7 @@ export default function InspectedElementSuspendedBy({
           maxTime={maxTime}
         />
       ))}
+      {unknownSuspenders}
     </div>
   );
 }
diff --git a/packages/react-devtools-shared/src/frontend/types.js b/packages/react-devtools-shared/src/frontend/types.js
index 7c7487b7bd6..61cc8b68fbf 100644
--- a/packages/react-devtools-shared/src/frontend/types.js
+++ b/packages/react-devtools-shared/src/frontend/types.js
@@ -19,6 +19,7 @@ import type {
   Unserializable,
 } from 'react-devtools-shared/src/hydration';
 import type {ReactFunctionLocation, ReactStackTrace} from 'shared/ReactTypes';
+import type {UnknownSuspendersReason} from '../constants';
 
 export type BrowserTheme = 'dark' | 'light';
 
@@ -281,6 +282,7 @@ export type InspectedElement = {
   suspendedBy: Object,
   // Minimum start time to maximum end time + a potential (not actual) throttle, within the nearest boundary.
   suspendedByRange: null | [number, number],
+  unknownSuspenders: UnknownSuspendersReason,
 
   // List of owners
   owners: Array<SerializedElement> | null,