Callinectes sapidus is native to the Atlantic coasts of the Americas. In the Mediterranean, it appeared around 1949 and though that it is established in East Mediterranean waters, relevant studies are limited. The aim of the present study is to report quantitative and qualitative data on the blue crab’s biology and ecology in its non-native range, that are indispensable for management purposes. Papapouli Lagoon is in Thermaikos Gulf and is ecologically impacted by the blue crabs. Fyke nets with a 20 mm mesh opening were soaked for 12 to 14 h during each survey. Abiotic environmental parameters were obtained. The length–weight relationships were expressed by the equation W = aLb. The species’ ecological preferences at Papapouli Lagoon were assessed with PERMANOVA analysis, using the abiotic parameters as factors. Student’s t-tests were used to assess the differences between sexes. The sex ratio of the blue crab’s population was assessed by a Chi-square (χ2) analysis. The sex ratio was found to be 1.28:1, in favour of males. The most dominant group size of male blue crabs was the 61–70 mm of CL and 130–139 mm of CW. Also, the dominant size group of female blue crabs was 60–69 mm of CL and 120–129 mm CW. The maximum abundance of blue crabs was recorded at a salinity range from 24‰ to 25‰ and the water temperature range was from 26 to 28 °C.