This study provided data on gastrointestinal morphology and feeding intensity of slender mudskipper Periophthalmus gracilis distributed in some coastal provinces in the Mekong Delta, Vietnam (VMD). Fish specimens were sampled monthly by hand-catching, from 01/2022 to 12/2022, in four provinces in the VMD: Tra Vinh, Soc Trang, Bac Lieu, and Ca Mau. After sexing, the fish total length and weight, gut length, and gastrointestinal morphology were measured and described. The study showed that Periophthalmus gracilis had a protruding upper jaw with a longer upper jaw bone length. The teeth are large and sharp, with pointed molars and unevenly distributed in a row on the upper jaw. The tongue was short and had a round tip. The gastrointestinal had an elastic tube-shaped esophagus, a short sac-shaped stomach, and long, straight, curving tube intestines. The feeding intensity of the mudskipper varied by season and month, with differences between males and females. These findings provided new information on the mudskipper’s gastrointestinal morphology and feeding habit.