We investigated how obligate mutualisms constrain species distributions under climate change, challenging the assumption that biotic interactions are negligible at macro-scales. By integrating host sea anemone distributions into Species Distribution Models for 17 anemonefish species, we found that host availability is a primary determinant of the realised niche, especially for specialists. Under future warming (SSP5-8.5), host immobility creates a biotic constraint, causing fish ranges to lag significantly behind their climatic potential. This mismatch generates over 3.2 million km² of climatically suitable but ecologically inaccessible ocean. Furthermore, specialist anemonefish species with the narrowest niches face the highest climate velocities while being constrained to the most dispersal-limited hosts. These findings indicate that climate-only assessments underestimate extinction risk. Conservation should shift to a host-first management strategy to prevent the collapse of these mutualisms.