The Antorun river is a flowing river situated at Onipanu village, along Ogbomoso-Iresaapa road, Nigeria. It was constructed primarily for irrigation purpose through the Nigerian-Canadian vegetation project and is under scrutiny for other purposes such as hydroelectric project, recreation, fishing and research. This study aimed at evaluating and documenting the species richness and diversity of the non-woody angiosperms along the reservoir. 1m2 quadrats were placed randomly at both rainy and dry seasons to sample non-woody angiosperms along upper (site A), middle (sites B and C) and lower (site D) courses which extend to about 1 km. A total of 1526 non-woody individual plants belonging to 28 genera and 13 families were identified in the study area. 22 of the plants were medicinal while others were forage plants. The family Asteraceae had the highest number of individuals (878) representing 9 species, with Tridax procumbens being the most frequent (11.3%). The importance Value Index (IVI) of the species in the study ranged between 1.1 and 44. Sampling point B recorded the highest diversity with Simpson’s index of 0.822 and Shannon-Wienner’s index of 1.849. The upper course of the reservoir was richest while the lower course was the poorest in species diversity and the commonest plant species in the study area was Emilia sonchifolia. Some woody species present in the study area were also identified and recorded. Information about plant diversity in this study area is useful in evaluating the potential impact of proposed developmental activities on the environment.