Glaucopsyche piasus, the arrowhead blue, is a western North American butterfly of the family Lycaenidae.

The upper side of males are violet blue with a wide border; females are duller.

The hindwing has a post-median band of white arrowheads pointing inwards.

The males patrol during the day near host plants, while females lay eggs on flower buds of the host plant.

Larvae feed on lupine (Lupinus) and milkvetch (Astragalus) species.