The present research introduces the construct of a decay theory of passion—a lay belief that romantic passion decline is irreversible—and investigates how this construct interacts with existing levels of passion for one’s romantic partner to predict lower relationship commitment and greater pursuit of romantic alternatives. Across three studies employing experimental and nonexperimental procedures—and a set of meta-analytic syntheses including additional studies—results generally supported the hypotheses that, although low passion is linked to lower commitment and greater pursuit of romantic alternatives, such effects are stronger when adherence to decay beliefs is high rather than low. These effects tended to be independent of effects of destiny and growth theories (Knee, 1998), a related set of lay theories in the domain of relationships. Mediated moderation analyses revealed that the moderating effect of decay theories on relationship commitment mediates the moderating effect of decay theories on the link between low passion and the pursuit of romantic alternatives. Discussion addresses the possibility that changing one’s beliefs surrounding the nature of romantic passion may be an important, but previously overlooked, means for preventing one from prematurely abandoning an otherwise satisfying relationship.