Theoretically, a shortage of males in a local marriage market may influence the formation, quality, and trajectory of unmarried parent relationships. To test these hypotheses, I combine city-level sex ratio data from the U.S. Census with microdata on unmarried couples who recently had a child from the Fragile Families study. A shortage of men in a marriage market is associated with lower relationship quality for unmarried parents. Male shortages are associated with lower rates of marriage following a nonmarital birth, and this is in part because of the mediating influence of relationship quality. A shortage of men is not significantly related to the economic quality of male, nonmarital childbearing partners.