The present study deals with water quality assessment and fish diversity during October-2023 to March-2024 in the Saryu River, Ayodhya, Uttar Pradesh; to find the correlation between fish biodiversity and water quality parameters. The result of the present study reveals the occurrence of 51 species of fish representing 34 genera and belonging to 17 families. The Cyprinid family was dominant, 37% belonged to this family, followed by the catfish family Bagridae (six species, three genera), and the remaining class in the present study. Various diversity indexes have been used to assess fish diversity. Fish diversity is also correlated with physicochemical variables. The Shannon Diversity Index for site 1 (Guptar Ghat), it varies from 2.187 to 3.17, and for site 2 (Dhemwa Ghat), it varies from 2.613 to 3.219. As per the comparison between the two sites, site 2 has lower values than site 1, which indicates that site 1 had higher fish biodiversity during the research period. The Simpson's Diversity Index Value It fluctuates between 0.7868 and 0.9539 for site 1 and between 0.8772 and 0.9585 for site 2. The Margalef index values for Site 1 and Site 2 range from 2.7 to 4.35 and 2.6 to 4.06, respectively. Greater species diversity is indicated by a higher Margalef index.