This study provides an understanding of the feeding habit and intensity of the mudskipper Periophthalmodon septemradiatus, which was a potential aquarium pet, by analyzing its relative gut length (RGL) and gastro-somatic (GaSI) indexes. Fish specimens were caught with fishing rods in an area from the Hau River’s estuary in Soc Trang province to its upstream in An Giang province during a period of one year from August 2017 to July 2018. The analysis of 1,504 fish samples shows that RGL did not change by fish size, resulting in no change in the feeding habit of P. septemradiatus in relation to its size. By contrast, the feeding habit of this mudskipper varied by site, month and season of sample collection; accordingly, the RGL was also significantly different by site, month and season of sample collection; both the male and female fish fell into the carnivorous as the RGL was P. Septemradiatus were not regulated by the interaction between fish size and site, fish size and season, and site and season. These results provide new knowledge on the feeding habit and intensity of this fish species, which also helps understand the fish’s adaption and conservation in the study area.