Current stocks cover only eight to nine days, raising concerns about the ability to meet medical needs in the near future.
Supplies of A negative and 0 negative are running dangerously low.
The Red Cross blames the shortage partly on a drop in donations over the hot summer months, which has not improved in recent weeks. Many regular donors are also unable to give blood at the moment because they have colds or the flu.
The Red Cross is urging all eligible people to donate in the coming days and weeks to rebuild reserves.
An appointment can be booked on dondusang.lu, or drop by spontaneously at the blood donation centre on 42, Boulevard Joseph II in Luxembourg City.
