Background: Body satisfaction interventions have typically been multifaceted and targeted at clinical populations. The aim of the present research was to isolate the effects of self-affirmation on body satisfaction in a community sample and to see whether self-affirmation works by basing one's self-esteem on domains other than body weight and shape. Methods: Adolescents (N = 220) were randomized to complete a self-affirmation manipulation or an equivalently active control task before rating their body shape and weight, and completing measures of perceived threat, body satisfaction and self-esteem. Results: Affirmed girls showed significantly greater body satisfaction and perceived significantly less threat from having to rate their body shape and weight compared with an equivalently active control group. Mediator analyses showed that the effects were due both to increases in self-esteem and shifts away from using body shape and weight as a source of self-esteem. Self-affirmation did not affect boys because they: (a) were less threatened by having to rate their body shape and weight, and (b) principally derived their self-esteem from sources other than body shape and weight. Conclusions: The findings provide support for the unique effects of self-affirmation on girls' body satisfaction thereby isolating one active ingredient of programs to increase body satisfaction and identify a potential mechanism for understanding self-affirmation effects. Further research is required to establish the long-term effects of self-affirmation and test how self-affirmation interacts with other active ingredients in treatment programs.