A mild palladium-catalyzed cross-coupling of unsubstituted and 2-alkyl-substituted aziridines with arylboronic acid nucleophiles is presented. The reaction is highly regioselective and compatible with diverse functionality. A catalytic amount of base, a sterically demanding triarylphosphine ligand, and a phenol additive are critical to the success of the reaction. Coupling of a deuterium-labeled substrate established that ring opening of the aziridine occurs with inversion of stereochemistry.