Size-based separation of platelets (1-3µm) from a background of RBCs (~7µm) in microfluidic devices is extremely challenging as the RBCs are highly deformable and can pass through gaps as small as 3µm. High shear stresses during sorting can activate platelets causing aggregation. We report a novel radial pillar device design that can enrich the platelet population by >25-fold. Our device can work with whole or diluted blood at low flow rates and can be used for ~8 hours without clogging.