diff --git a/internal/api/me_notifications.go b/internal/api/me_notifications.go
index 885e816d..8c7ae485 100644
--- a/internal/api/me_notifications.go
+++ b/internal/api/me_notifications.go
@@ -3,6 +3,7 @@ package api
 import (
 	"encoding/json"
 	"errors"
+	"io"
 	"net/http"
 	"strconv"
 	"strings"
@@ -170,7 +171,20 @@ func (h *handlers) handleMarkAllMyNotificationsRead(w http.ResponseWriter, r *ht
 	if !ok {
 		return
 	}
-	if _, err := dbq.New(h.pool).MarkAllNotificationsRead(r.Context(), user.ID); err != nil {
+	// Optional {"up_to": RFC3339}: only what existed then. A client replaying
+	// an offline "mark all read" sends the moment the user asked.
+	var body struct {
+		UpTo *time.Time `json:"up_to"`
+	}
+	if err := json.NewDecoder(r.Body).Decode(&body); err != nil && !errors.Is(err, io.EOF) {
+		writeErr(w, apierror.BadRequest("bad_body", "invalid JSON body"))
+		return
+	}
+	params := dbq.MarkAllNotificationsReadParams{UserID: user.ID}
+	if body.UpTo != nil {
+		params.UpTo = pgtype.Timestamptz{Time: *body.UpTo, Valid: true}
+	}
+	if _, err := dbq.New(h.pool).MarkAllNotificationsRead(r.Context(), params); err != nil {
 		writeErrWithLog(w, h.logger, "notifications: mark all read", apierror.Internal(err))
 		return
 	}
diff --git a/internal/api/me_notifications_test.go b/internal/api/me_notifications_test.go
index 3bf34110..1839b3ab 100644
--- a/internal/api/me_notifications_test.go
+++ b/internal/api/me_notifications_test.go
@@ -7,6 +7,7 @@ import (
 	"net/http"
 	"net/http/httptest"
 	"testing"
+	"time"
 
 	"github.com/go-chi/chi/v5"
 	"github.com/jackc/pgx/v5/pgxpool"
@@ -127,6 +128,33 @@ func TestMyNotifications_MarkReadIsOwnerScopedAndCountsDown(t *testing.T) {
 	}
 }
 
+// A "mark all read" replayed from an offline queue carries the moment the
+// user asked; what arrived after it stays unread.
+func TestMyNotifications_ReadAllUpToLeavesLaterOnesUnread(t *testing.T) {
+	h, pool := testHandlers(t)
+	alice := seedUser(t, pool, "notif-upto", "pw", false)
+	r := notificationsRouter(h)
+
+	notifyN(t, pool, alice, 1)
+	var page notificationsPageResp
+	callAs(t, r, alice, http.MethodGet, "/api/me/notifications", "", &page)
+	seen := page.Items[0].CreatedAt
+	notifyN(t, pool, alice, 1)
+
+	body := `{"up_to":"` + seen.Format(time.RFC3339Nano) + `"}`
+	if code := callAs(t, r, alice, http.MethodPost, "/api/me/notifications/read-all", body, nil); code != http.StatusNoContent {
+		t.Fatalf("read-all up_to = %d", code)
+	}
+	var count unreadCountResp
+	callAs(t, r, alice, http.MethodGet, "/api/me/notifications/unread-count", "", &count)
+	if count.UnreadCount != 1 {
+		t.Errorf("unread = %d, want 1 (the one that arrived later)", count.UnreadCount)
+	}
+	if code := callAs(t, r, alice, http.MethodPost, "/api/me/notifications/read-all", `{"up_to":"yesterday"}`, nil); code != http.StatusBadRequest {
+		t.Errorf("malformed up_to = %d, want 400", code)
+	}
+}
+
 func strPtr(s string) *string { return &s }
 
 func setSMTP(t *testing.T, pool *pgxpool.Pool, enabled bool) {
diff --git a/internal/db/dbq/notifications.sql.go b/internal/db/dbq/notifications.sql.go
index 82e43f42..2c12a44a 100644
--- a/internal/db/dbq/notifications.sql.go
+++ b/internal/db/dbq/notifications.sql.go
@@ -214,11 +214,22 @@ func (q *Queries) ListNotifications(ctx context.Context, arg ListNotificationsPa
 const markAllNotificationsRead = `-- name: MarkAllNotificationsRead :execrows
 UPDATE user_notifications
    SET read_at = now()
- WHERE user_id = $1 AND read_at IS NULL
+ WHERE user_id = $1
+   AND read_at IS NULL
+   AND ($2::timestamptz IS NULL OR created_at <= $2)
 `
 
-func (q *Queries) MarkAllNotificationsRead(ctx context.Context, userID pgtype.UUID) (int64, error) {
-	result, err := q.db.Exec(ctx, markAllNotificationsRead, userID)
+type MarkAllNotificationsReadParams struct {
+	UserID pgtype.UUID
+	UpTo   pgtype.Timestamptz
+}
+
+// up_to, when set, limits it to what existed when the user asked: a "mark
+// all read" queued offline and replayed later must not mark notices that
+// arrived in between, which the user never saw. A coalesced row updated
+// since then carries a newer created_at, so it stays unread too.
+func (q *Queries) MarkAllNotificationsRead(ctx context.Context, arg MarkAllNotificationsReadParams) (int64, error) {
+	result, err := q.db.Exec(ctx, markAllNotificationsRead, arg.UserID, arg.UpTo)
 	if err != nil {
 		return 0, err
 	}
diff --git a/internal/db/queries/notifications.sql b/internal/db/queries/notifications.sql
index 8abc10fe..2779f728 100644
--- a/internal/db/queries/notifications.sql
+++ b/internal/db/queries/notifications.sql
@@ -57,9 +57,15 @@ UPDATE user_notifications
  WHERE id = sqlc.arg(id) AND user_id = sqlc.arg(user_id);
 
 -- name: MarkAllNotificationsRead :execrows
+-- up_to, when set, limits it to what existed when the user asked: a "mark
+-- all read" queued offline and replayed later must not mark notices that
+-- arrived in between, which the user never saw. A coalesced row updated
+-- since then carries a newer created_at, so it stays unread too.
 UPDATE user_notifications
    SET read_at = now()
- WHERE user_id = $1 AND read_at IS NULL;
+ WHERE user_id = sqlc.arg(user_id)
+   AND read_at IS NULL
+   AND (sqlc.narg(up_to)::timestamptz IS NULL OR created_at <= sqlc.narg(up_to));
 
 -- name: TrimNotifications :execrows
 -- Retention: read rows go after read_cutoff, and anything at all after
diff --git a/internal/library/notify_test.go b/internal/library/notify_test.go
index eb7b4ba9..0ca635e2 100644
--- a/internal/library/notify_test.go
+++ b/internal/library/notify_test.go
@@ -151,7 +151,7 @@ func TestDuplicateSweep_NotifiesOnlyWhenSomethingNewIsProposed(t *testing.T) {
 	}
 	markAllRead := func() {
 		t.Helper()
-		if _, err := q.MarkAllNotificationsRead(ctx, admin.ID); err != nil {
+		if _, err := q.MarkAllNotificationsRead(ctx, dbq.MarkAllNotificationsReadParams{UserID: admin.ID}); err != nil {
 			t.Fatalf("mark read: %v", err)
 		}
 	}