Iron and temperature are important drivers controlling phytoplankton growth in the Southern Ocean. Most studies examining phytoplankton responses to these variables consider them independently, testing responses to changing temperature under constant iron and vice versa. Consequently, we lack a phenomenological and mechanistic understanding of how concurrent changes in these variables influence primary productivity. Here we used a matrix of three temperatures and eight iron levels to examine changes in growth rate, photophysiology and size in Fragilariopsis cylindrus over four generations. Temperature and iron interactively influenced growth; warming decreased iron demand, allowing cells to maintain half-maximal growth rate at lower iron concentrations. We also observed possible mechanisms underpinning this phenomenon: warming increased light-harvesting cross section and reduced cell size, thereby increasing light energy availability and iron uptake efficiency. These results suggest interactive iron-warming effects could lead to larger increases in S.O. phytoplankton growth than those currently predicted by marine ecosystem models.