College women are at risk for body dissatisfaction, but enhancing the positive aspects of body image can serve as a protective factor. Self-compassion has been associated with body appreciation and may protect against negative body image. This study tested the effect of a self-compassion writing intervention on positive body image and affect and explored self-compassion as a mediator between writing group and positive body image. We used a randomized controlled design to compare self-compassion writing (n = 51), traditional expressive writing (n = 50), and control writing (n = 51) interventions in college women. Participants were mostly European-American (82%) with a mean age of 19. Participants wrote online for 20 min once a week for three consecutive weeks. Results indicated that negative and positive affect decreased for all three groups. There were no differences between groups on positive body image or affect; however, the self-compassion writing group reported greater increases in self-compassion (F = 3.48, p < 0.05, ηp2 = 0.05). Moreover, mediator models revealed that the effect of group (self-compassion vs. traditional/control writing) on body appreciation and body image quality of life was mediated by self-compassion. Overall, the findings indicate that self-compassion writing increased self-compassion, and greater increases in self-compassion were associated with greater increases in positive body image and positive affect.