Caragobius urolepis thrives in tropical brackish and freshwater regions, including Vietnam’s Mekong Delta (MD), where it is commonly caught for food. However, little data are known on this fish, especially feeding habits and diet composition used for fish ecological adaptation and controlled breeding research. Thus, this study was conducted at two coastal locations in MD to provide data on this fish. Data analysis of 628 specimens collected monthly by trawl nets from October 2022 to September 2023 revealed that this fish is a carnivore, as indicated by its relative gut length (RGL) being less than 1. Feeding habits varied with season, as RGL in the dry season (0.36 ± 0.00; n = 296) was higher than in the wet season (0.34 ± 0.00; n = 332) (PERMANOVA, p = 0.002). The RGL of this fish also displayed a spatial variation (p = 0.03). Analysis of 272 individuals (147 males, 125 females) containing food in the digestive tract of 628 specimens showed that the fish diet consisted of Polychaeta (13.45%), mollusks (24.04%), small fish (26.23%), Acetes spp. (23.06%) and Uca spp. (13.31%). The diet spectrum varied seasonally (p < 0.05), with Uca spp. contributing to this difference (Mann-Whitney U, p < 0.05). The findings enhance the understanding of the fish’s ecological adaptations, serving as a foundation for future breeding research and contributing to the region’s conservation efforts and the diversification of cultured fish.