Dimorphisms occur when alternative developmental pathways produce discrete phenotypes within a species, and may promote evolutionary novelty in morphology, life history, and behavior. Among marine invertebrates, intra-specific dimorphism in larval type (poecilogony) is notably rare, but should provide insight into the selective forces acting on larval strategies. Most established cases of poecilogony appear to be allelic polymorphisms, with local expression regulated by population-genetic processes. Here, we present evidence that dimorphic larval development in the sea slug Alderia willowi is a seasonal polyphenism; the type of larvae produced by an adult slug depends on the rearing environment in which that slug matured. In field surveys of 1996–1999 and 2007–2009, the population in Mission Bay, San Diego (California, USA) produced only short-lived lecithotrophic larvae in summer and early fall, but a varying proportion of slugs expressed planktotrophy in winter and spring. In laboratory experiments, slugs reared under summer conditions (high temperature, high salinity) produced the highest proportion of lecithotrophic offspring, whereas winter conditions (low temperature, low salinity) induced the lowest proportion of lecithotrophy. The shift to a nondispersive morph under summer conditions may be an adaptive response to historical closure of coastal wetlands during the dry season in southern California, which would inhibit dispersal by larvae of back-bay taxa. In most animal polyphenisms, a single larval type is produced and the rearing environment determines which adult phenotype develops. In contrast, alternative larval morphs are produced by A. willowi in response to seasonal cues experienced by the adult stage, varying the phenotype and dispersal potential of offspring. As the only known case of polyphenism in mode of larval development, A. willowi should become a model organism for mechanistic studies of dimorphism and the evolution of alternative life histories.