The blue swimming crab, Portunus segnis is a Lessepsian migrant into the Mediterranean and little is known about its biology in Iskenderun Bay, North-eastern Mediterranean, Turkey. In this study, we examined the sex ratio, gonadal development stages, gonadosomatic index, size at first sexual maturity, and fecundity of P. segnis. The specimens of P. segnis were collected monthly from July 2014 to June 2015 (except February) using a trawl net from Yumurtalik Cove. The crabs were found throughout the year and were abundant from July to October (74%). Carapace width (CW) range of all samples was 38.1-163.2 mm, and body weight ranged from 3.5 to 324.4 g. The population's sex ratio was Male: Female=0.7:1, which shows that the female population is more abundant. Fifty percent of the female crabs attained sexual maturity when they reached the size of 115-120 mm CW. The ovigerous crabs ranging between 101.4 and 154 mm CW, and 88.3-324.4 g in body weight can produce 139379 to 2745236 eggs. The mean fecundity of P. segnis was 1070425 +/- 580978 eggs with mean carapace width of 130.0 +/- 12.8 mm and mean body weight 191 +/- 59 g. The crabs produced 5607.8 +/- 2559.9 eggs (mean +/- sd) per gram of body weight. In Yumurtalik Cove, breeding of P. segnis was observed throughout the year, except in winter. Further studies are needed to assess the reproductive biology of blue crabs in other locations of Turkey.