Although expressive writing has positive effects on health, little is known about the underlying psychological mechanisms for these effects. The present study assessed self-affirmation, cognitive processing, and discovery of meaning as potential mediators of the effects of expressive writing on physical health in early-stage breast cancer survivors. A content analysis of the essays showed that self-affirmation writing was associated with fewer physical symptoms at a 3-month follow-up assessment, with self-affirmation writing fully mediating the effects of the emotional expression and benefit-finding writing conditions on reduced physical symptoms. Cognitive processing and discovery of meaning writing were not associated with any physical health outcomes. Consistent with evidence showing that self-affirmation plays an important role in buffering stress, the present study provides the first evidence for self-affirmation as a viable mechanism underlying the health benefits of expressive writing.