Objective: In this study carried out in Büyükçekmece Reservoir, the composition, distribution, seasonal changes of epiphytic diatoms that live on the surfaces of plants, and the effects of environmental parameters on these organisms were investigated, and it was aimed to reveal the water quality of the reservoir. Materials and Methods:In order to determine the seasonal changes of epiphytic diatom species in Büyükçekmece Reservoir, water and material samples were collected from five stations in 2019. In the study, water temperature, salinity, conductivity, dissolved oxygen, and pH values, which are among the basic ecological variables, were measured. Epiphytic diatom samples were obtained from Phragmites sp. species. Also, Spearman's rank correlation, Shannon-Weaver diversity index, Cluster (Bray-Curtis and Euclidean Distance), and ordination analysis (DCA and CCA) were applied in the study.Results: 66 epiphytic diatom species were identified in this study, and 36 of these species are new records for the reservoir. Most epiphytic diatom species were obtained in August, and the lowest number was obtained in November. According to the pH values, it was determined that the reservoir is alkaline. In addition, it was determined that the main factors affecting the distribution of epiphytic diatom species in the reservoir are temperature and conductivity, and it was revealed that ecological variables affect species distribution. Conclusion:According to conductivity and DO values, it was determined that the reservoir was of very good and of good quality, and in terms of DO values, the reservoir was mainly oligotrophic. However, station 5 was mesotrophic during the August sampling period, station 4 was mesotrophic, and station 5 was eutrophic in November. Also, Büyükçekmece Reservoir was found in poor and moderate status according to H' classification.