Glossogobius is one of the largest genera of the family Gobiidae with 29 species. Of them, three species including G. giuris, G. aureus, and G. sparsipapillus have been considered important species in the Mekong Delta, Vietnam. Until now, the systematic study of these species has been limited. This study aims to provide new knowledge on relative gut length (RGL) and fullness index (FI), and on the variation of these two indexes with sex, size, site and season. The experiment was carried out from January to December 2020 at four provinces in the Mekong Delta: Can Tho, Soc Trang, Bac Lieu, and Ca Mau. 1291 specimens (659 females and 632 males) were captured. Data analyses show that the goby belongs to the carnivorous category as the RGL is lower than 1. There is a variation of RGL and FI between two biometric indexes and saturation coefficient of G. giuris. This variation depends not only on the genotype of the fish but also on environmental conditions such as season and site. There are no differences of RGL and FI between females and males, and immature and mature fish, except for FI, where differences are observed between immature and mature individuals. There are significant differences in RGL and FI between seasons. The RGL had a maximum value in the wet season, while the FI had a high value in the wet season. The RGL varies from one site to another. The maximum value is at Dam Doi, Ca Mau province and lower values are observed for fish from Cai Rang, Can Tho and Hoa Binh, Bac Lieu province. The FI value varies from the maximum value in Cai rang, Can Tho to a minimum value in Long Phu, Soc Trang province. The variations of RGL and FI of this fish are regulated by sex x site, and season x site interactions. The findings help better understand the fish feeding habitat and intensity, being used to evaluate the fish adaption.