Maduru Oya National Park (MONP) including its largest reservoir, Maduru Oya Reservoir (MOR), has been identified with a high waterbird density. However, a limited number of researches have been conducted on waterbirds in MONP. In this study, diversity, habitat use of waterbirds at MOR and their nesting characteristics were analyzed. Bird survey was carried out using the point count method and block count method. A common ethogram was constructed to identify the behavior categories. Percentage cover of habitat types was estimated by quadrat method. A total of 30 species belonging to 15 families were recorded, including two globally threatened species and three winter visitors. Year around Shannon-Weiner index for MOR was 1.491. Little Cormorant had the highest relative abundance. Percentage coverage of grass had increased gradually from March to September with the highest in September (69.27%) while open water had the highest coverage in the other months. Significantly high number of individuals used areas of grass, open water and mud for feeding and exposed rocks, dead trees/logs for resting (One-way ANOVA, p ≤ 0.05). Four waterbird species were observed building nests. Occupied nesting tree species were Senna spectabilis (Kaha Kona), Dymorphocalyx glabellus (Weliwanna) and Alstonia scholaris (RukAththana). The number of nests observed in three layers of canopy was significantly different (p ≤ 0.05). The highest average number of nests was observed in middle layer (8.90 ± 2.67). Black-headed Ibises had high preference to nest in Dymorphocalyx glabellus (p ≤ 0.05, 3.30 ± 1.32) while Great Cormorants (p ≤ 0.05, 2.38 ± 0.74) and Grey Herons (p ≤ 0.05, 4.27 ± 2.12) in Alstonia scholaris. Little Cormorants had no significant difference in their preference for nesting trees. As a preliminary study, this can be used for future research on