Nigella L. (Ranunculaceae) is distributed worldwide and is primarily centered in the Mediterranean region. Fourteen species are reported from Turkey, 10 of which are native. Specimens of the new species, N. koyuncui Dönmez & Uğurlu, were collected from Sinop, Turkey. It is distinguished from N. arvensis L. s.l. by several characters, including the dark red to purplish red central blotch of flowers, the whitish yellow sepals, yellow petals, large nectary pairs, and grouped stamens. Chromosomes of the new species have been studied for the first time, and the diploid number (2n) has been found to be 12.