Here we present evidence of seasonal reproduction in the deep-sea vent mussel Bathymodiolus azoricus, a dominant member of the Mid-Atlantic Ridge (MAR) hydrothermal fauna in the Azores region. This is the first time that seasonal reproduction has been suggested for any deep-sea vent organism. This discovery was made possible by the use of novel, acoustically-retrievable cages, which allowed us to extend the frequency and temporal range of sampling that was previously limited to the summer months. The main spawning peak, at the Menez Gwen vent field (840 m) occurs in late December–January and shows a correlation with a winter–spring bloom in primary production in the euphotic zone. Our results suggest that this surface-derived material may act as both a food source for the dispersing mussel larvae and as a reproductive cue/supplementary nutritional source for the adult mussels. Further evidence of a dependence on photosynthetic inputs comes from the relationship between particulate feeding levels, which themselves correlate with the phytoplankton peak, and the amounts of storage tissue in the mantle, which ultimately gets converted into gonad. Thus, the pattern and energetics of reproduction in the Atlantic vent mussel B. azoricus closely resembles that found in the coastal mussel Mytilus edulis, which has been described as an adaptation for optimizing the timing of reproduction against a background of seasonally-varying food availability. This discovery emphasizes the complexity of the nutritional pathways found in some deep-sea chemosynthetic environments and highlights the need for more time-series studies.