Abstract Ammonia (NH3) measurements are challenging to make, and air‐sea flux estimates are sparse, with coastal observations complicated by overlapping marine and terrestrial signals. Here we present high‐resolution NH3 air‐sea flux estimates based on two years (2022–2023) of atmospheric and seawater measurements at the Penlee Point Atmospheric Observatory (PPAO), on the southwest UK coast. NH3 concentrations peak in spring and summer and were strongly influenced by terrestrial air masses from the UK and Continental Europe. Net NH3 deposition occurred across all seasons, with seasonal median deposition ranging from 20.5 to 34.5 μmol m−2 d−1 or 1.0 to 1.8 kg NH3‐N ha−1 yr−1. The flux direction is primarily controlled by atmospheric NH3, while seasonal changes in seawater biogeochemistry have a secondary role. These findings highlight the role of anthropogenic NH3 in coastal nitrogen budgets and underscore the need for high‐resolution flux estimates.