I examined sexual size dimorphism (SSD) and assortative mating and estimated a discriminant function for predicting sex in the Whiskered Tern Chlidonias hybrida, a medium-sized marsh tern. I measured and molecularly sexed 241 breeding adults captured in southern Poland from 2005 to 2010. Males were significantly larger than females in all six measurements: total head length, bill length, bill depth at gonys, tarsus length, wing length and body mass. The largest dimorphism index (DI) was found in head and bill measurements and body mass. Within pairs, the total head length, bill length and bill depth were positively correlated. Two discriminant functions, one based on the length and depth of the bill and the second including only total head length, showed the same high sex classification success of 95%. DI in Whiskered Terns was greater than in other terns probably due to stronger sex specialization in foraging niche.