diff --git a/backend/internal/sync/service.go b/backend/internal/sync/service.go
index 4d3297d..b1c4a82 100644
--- a/backend/internal/sync/service.go
+++ b/backend/internal/sync/service.go
@@ -7,6 +7,7 @@ package sync
 import (
 	"context"
 	"encoding/json"
+	"errors"
 	"fmt"
 	"log"
 	"sync"
@@ -363,7 +364,18 @@ func (s *Service) fillPendingWorkouts(ctx context.Context, limit int) error {
 			}
 		}
 		if err := s.fillActivityWorkout(ctx, a, profile); err != nil {
-			log.Printf("sync: fill workout for activity %d failed, will retry next sync: %v", a.GarminActivityID, err)
+			if errors.Is(err, garmin.ErrNotFound) {
+				// A definitive 404 (the workout was deleted on Garmin's side
+				// after being linked to this activity) will never succeed on
+				// retry -- mark it so ActivitiesMissingWorkout stops
+				// surfacing it, instead of retrying forever.
+				log.Printf("sync: workout for activity %d not found on Garmin, marking as such (will not retry): %v", a.GarminActivityID, err)
+				if serr := s.db.SetActivityWorkoutNotFound(ctx, s.userID, a.ID); serr != nil {
+					return fmt.Errorf("mark activity %d workout not found: %w", a.GarminActivityID, serr)
+				}
+			} else {
+				log.Printf("sync: fill workout for activity %d failed, will retry next sync: %v", a.GarminActivityID, err)
+			}
 		}
 		s.setProgress(PhaseWorkouts, i+1, len(pending))
 	}
diff --git a/backend/internal/sync/service_test.go b/backend/internal/sync/service_test.go
index 236ad4d..ae6f92a 100644
--- a/backend/internal/sync/service_test.go
+++ b/backend/internal/sync/service_test.go
@@ -902,6 +902,68 @@ func TestFillPendingDetails_OneWorkoutFetchFailureDoesNotAbortTheWorkoutsPass(t
 	}
 }
 
+func TestFillPendingDetails_NotFoundWorkoutStopsBeingRetried(t *testing.T) {
+	db := openTestDB(t)
+	ctx := context.Background()
+	userID := provisionTestUser(t, db)
+
+	const missingWorkoutID = 300
+	missingPtr := int64(missingWorkoutID)
+	m := &mock.Client{
+		Activities: []garmin.Activity{
+			{ActivityID: 1, ActivityType: garmin.ActivityType{TypeKey: "running"}, WorkoutID: &missingPtr,
+				StartTimeGMT: "2026-07-01 06:00:00", Distance: 5000, Duration: 1500},
+		},
+		Splits: map[int64]garmin.ActivitySplits{
+			1: {ActivityID: 1, Laps: []garmin.Lap{{LapIndex: 1, IntensityType: "ACTIVE"}}},
+		},
+		Details:        map[int64]garmin.ActivityDetails{1: {ActivityID: 1}},
+		Workouts:       map[int64]garmin.Workout{},
+		WorkoutErrByID: map[int64]error{missingWorkoutID: fmt.Errorf("API Error 404: %w", garmin.ErrNotFound)},
+	}
+	svc := NewService(m, db, userID, Config{InterCallDelay: time.Millisecond},
+		fixedNow(time.Date(2026, 7, 11, 0, 0, 0, 0, time.UTC)))
+
+	if _, err := svc.backfillCore(ctx); err != nil {
+		t.Fatalf("backfillCore: %v", err)
+	}
+	if err := svc.FillPendingDetails(ctx, 10); err != nil {
+		t.Fatalf("FillPendingDetails should not return an error for a confirmed-404 workout: %v", err)
+	}
+
+	remaining, err := db.CountActivitiesMissingWorkout(ctx, userID)
+	if err != nil {
+		t.Fatalf("CountActivitiesMissingWorkout: %v", err)
+	}
+	if remaining != 0 {
+		t.Fatalf("CountActivitiesMissingWorkout = %d, want 0 (confirmed-404 activity must not be retried)", remaining)
+	}
+
+	activities, err := db.ListActivities(ctx, userID, store.ActivityFilter{})
+	if err != nil {
+		t.Fatalf("ListActivities: %v", err)
+	}
+	if len(activities) != 1 {
+		t.Fatalf("ListActivities returned %d activities, want 1", len(activities))
+	}
+	got, _, err := db.GetActivity(ctx, userID, activities[0].ID)
+	if err != nil {
+		t.Fatalf("GetActivity: %v", err)
+	}
+	if got.WorkoutNotFoundAt == nil {
+		t.Error("WorkoutNotFoundAt is nil, want it set")
+	}
+	if got.WorkoutRawJSON != nil {
+		t.Error("WorkoutRawJSON should stay nil -- not-found is recorded separately, not faked")
+	}
+
+	// A second FillPendingDetails call must not attempt this workout again
+	// (it's no longer in ActivitiesMissingWorkout's result set at all).
+	if err := svc.FillPendingDetails(ctx, 10); err != nil {
+		t.Fatalf("second FillPendingDetails: %v", err)
+	}
+}
+
 func TestService_TwoUsersSyncIndependently(t *testing.T) {
 	db := openTestDB(t)
 	ctx := context.Background()