Length-weight relationship (LWR) parameters are necessary to understand the life cycle of fish to describe the biological aspects of species. In this sense, LWRs were determined for 17 estuarine species from the second largest mangrove area in the world, Northern Brazil. The sampling was carried out during the dry and wet seasons of 2015 and 2018 in 3 estuaries described as conservation units in the rivers Pará, Caeté, and Parnaíba. Fish were collected by using manual beach seine, block nets and sieves. Overall 488 specimens were analyzed using the equation W = a TLb. The a values ranged from 0.000001 to 0.000019 and b values ranged from 2.7230 to 3.2821. This study brings new data on estuarine fish from Brazil with additional information contributing to the conservation and management of the fish stocks.