The alien blue swimming crab, Portunus segnis (Forskål, 1775), an Indo-Pacific species, has been recorded in the southern Mediterranean Sea for decades, and its occurrence along the southern Tunisian coasts has been recorded since 2014. The feeding habits of P. segnis were studied in the Gulf of Gabes from October 2015 to September 2016. The stomach contents of 3061 crabs, ranging from 50 to 168 mm carapace width, were analysed. The stomach contents appeared to consist mainly of large quantities of crustaceans, fish and molluscs, and small quantities of echinoderms, annelids, tunicates, cnidarians, plants, unidentifiable matter and debris. The major food groups observed were crustaceans throughout the year with fish and molluscs secondly. In juvenile, sub-adult and adult crabs, crustaceans constituted the dominant food source and these were present in more than 65% of the stomachs analysed. Ingestion of fish increases as the crab matures. A significant difference (P < 0.05) was observed in the quantity of the food consumed by the two sexes. The diet also increased with increasing ovary maturity degree, while the percentage of empty stomachs in ovigerous and non-ovigerous females did not show any difference. The vacuity index varied significantly in both sex (P < 0.0001), among size (P < 0.0001) and seasons (P < 0.0001). This study shows that, despite the diversity in blue crab diets and feeding habits, it is omnivorous with a preference for animal material.