The rarest blood type is AB-negative (AB-). It’s found in less than 1% of the global population, with estimates around 0.6% in the U.S., based on data from the American Red Cross. The rarity comes from the combo of the AB blood group (already uncommon) and the negative Rh factor, which only about 15% of people lack. It’s super valuable for transfusions since AB- folks can only receive AB- blood, though their plasma can be donated to anyone.