The impact of keystone predators may be affected by physical conditions and interactions with other predators. We evaluated the impact of the sea star Meyenaster gelatinosus on another sea star, Heliaster helianthus, which was previously described as a keystone predator along the coast of central Chile. Field surveys showed that H. helianthus is a common prey of M. gelatinosus. H, helianthus occurred across the subtidal zone at locations with few M gelatinosus, but was largely restricted to the intertidal and upper-subtidal zones at locations where M. gelatinosus was abundant. The proportion of H. helianthus feeding decreased with increasing M gelatinosus abundance, and H. helianthus with autotomized arms, mainly the result of sublethal attacks by M. gelatinosus, increased with increasing M gelatinosus abundance. Additions of M gelatinosus to a sediment bottom area where H, helianthus was actively feeding on the gastropod Turritella cingulata provoked (1) reduced feeding and (2) a strong escape response in which H. helianthus took the 'crown' position (arm tips raised) in about 40 s and then fled. T cingulata is an abundant prey resource that is likely only available to H. helianthus when M gelatinosus is rare. The predatory role of H. helianthus is greatly reduced when M. gelatinosus is common. Our observations suggest that M gelatinosus, rather than H. helianthus or the whelk Concholepas concholepas, is the keystone predator in subtidal communities. Our study demonstrates that the role of a keystone predator (in our case H. helianthus in the intertidal zone) can change in an adjacent habitat (the subtidal zone) due to interactions with other predators.