Little is known on growth and size relationships of Glossogobius olivaceus, a high-value commercial species in northern Vietnam. This study aimed to understand morphometric relationships, growth and condition factor of G. olivaceus from mangrove forests of the Ba Lat estuary, the Red River in northern Vietnam. Analysis of 679 G. olivaceus collected monthly from March 2018 to February 2019 showed a sex ratio of approximately 1:1. The mean total length and body weight were not significantly different between females and males. Length-weight relationships (LWR) for G. olivaceus showed high correlations that varied slightly by season. This species presented a positive allometric growth pattern as the slope b of the LWR which was significantly higher than the cubic value of 3. Estimates of condition factor (K) were not different from the value of 1, implying a favorable nutritional condition of specimens collected. Both the LWR (b slope) and condition factor (K) of G. olivaceus from our study varied by sex, and seasons indicated that G. olivaceus lives across variable environmental conditions. Furthermore, shifts in growth patterns between the breeding (b ≈ 3) and non-breeding season (b > 3) potentially suggest that this species has an adaptation strategy to monthly/seasonal environmental variability.