Low self-esteem is an important factor associated with body dysmorphic concerns. In treatment, self-esteem cannot always be adequately addressed. Internet-based interventions offer a low-threshold and cost-efficient possibility for treating body dysmorphic disorder (BDD). For this reason, we conducted two studies to explore the effectiveness of an internet-based intervention targeting improving self-esteem in adults with BDD symptoms. The first study investigated the differential effects of a 1-week self-esteem training compared with a 1-week attention-focus training. Two hundred twenty adults with elevated body dysmorphic symptoms were randomly assigned to one of the two trainings. Our second study (n = 58 adults with body dysmorphic symptoms) evaluated an extended 2-week stand-alone self-esteem training. In the first study, self-esteem in different domains (appearance, performance and social), self-focused attention, and BDD symptom severity improved in both groups. Other-focused attention only increased in the attention training group. Participants' overall adherence was high. In the second study we observed significant improvements in self-esteem, BDD symptom severity, and other secondary outcomes, with additional improvements in most outcomes in the second week. Adherence was again high. Together, these findings show that a brief internet-based intervention may be a highly accepted and effective way of improving self-esteem in people suffering from BDD symptoms.