The rate of overexploitation, pollution, habitat destruction, climate change, invasive species, and disease outbreaks in inland waters pose significant threats to biodiversity, underscoring the need for comprehensive research on freshwater ecosystems. Monitoring the diversity, abundance and richness of species in these ecosystems is crucial for assessing their status and stability. This research aimed to develop a comprehensive catalogue of fish species in Omi-Nla River, Agbabu. The catalogue will provide a vital baseline for conservation and management efforts, promoting the long-term sustainability of the river's ecosystem. This research was carried out in Omi-Nla River, for eight months from January,2025 to August 2025, in three fishing communities (station 1, Station 2, and station 3) based on their level of fishing activity. Data were collected through experimental field record of fish species across three landing sites with each site sampled on monthly basis with minimum of three canoes assessed in each location for their catches. The catches were sorted and identified into species using standard fish identification keys. Results revealed that total of 4,143 individuals were identified across (11) families, the Cyprinidae family has the highest species richness comprises of (11) species and Polyteridae family were the least diverse. Clarias anguilaris species was the most prevalent accounting for 7.57% of the total population with 241 individuals while, the least abundant species were Chrysicthys walker (1.04%). The highest Dominance (D) was recorded for Midstream station (1248) least was recorded for Upstream station (1048) Shannon (H) recorded the highest for Downstream (3.39) while the least was recorded for Upstream (3.32), evenness (E) recorded the highest for Downstream (10.76) and the least for the Upstream (0.71) Margalef (F) recorded the highest for Upstream (5.46) and the least for Midstream (5.33).This study provides foundational data sources that could be used by relevant agencies for conservation and management of fisheries resources of Omi-Nla River, Agbabu.