The Black-naped Tern (Sterna sumatrana) breeds throughout the Indian and Western Pacific Oceans. Given the comparative rarity of the mathewsi subspecies, a better understanding of its population dynamics is needed. This study aims to improve knowledge of Black-naped Tern breeding ecology and contribute to regional population assessment. Data was collected at the Alphonse Group, Seychelles, where between 2008 and 2022 surveys recorded; abundance of adults, breeding pairs, productivity, location, and seasonality. Counts of maximum adults ranged from 38 to 62 (50.53, SD 7.29), displaying a slight positive trend, while breeding pairs numbered 11 to 28 (16.98, SD 5.04), showing a decrease. Estimates of minimum and maximum productivity were 0.00 to 0.75 (0.23, n = 21) and 0.00 to 1.88 (0.49, n = 21), respectively, and showed a significant relationship with site and season. Results indicate reproductive success and breeding site preference is becoming increasingly variable. Breeding was temporally widespread, with two discernable peaks in activity in February and June. Changing environmental conditions are the likely cause of breeding pair declines and increasing variability of; inter-annual productivity, breeding onset and site-selection. Findings highlight this species vulnerabilities, the importance of consistent monitoring, and enforced protective measures such as capped visitor numbers and seasonal closure of breeding sites.