Background and Objective: Body dysmorphic disorder (BDD), known as an obsessive-compulsive disorder, is considered a risk factor for mental health and well-being, especially during adolescence and particularly among adolescent girls. Body image anxiety and suicidal ideation are two categories of complications that have been linked to BDD. The present study aimed to investigate the effect of mindfulness-based cognitive therapy on body image anxiety and suicidal ideation of adolescent girls with BDD. Materials and Methods: This study was a quasi-experimental longitudinal study in the form of a pretest-posttest design with experimental and control groups. It was conducted from April 9, 2020, to August 18, 2020. The participants included 25 adolescent girls with a BDD score of more than 25, who were randomly selected from among the adolescent girls studying in schools of District 5 in Tehran, Iran. Data were analyzed in the SPSS software (version 22) using the one-way ANCOVA test. Results: The findings revealed that by removing the effect of pre-test scores as a covariate variable, the main effect of treatment on post-test scores (is significant for body image anxiety (P<0.01), determined at 16%. The findings also showed that by eliminating the effect of pre-test scores as a covariate variable, the main effect of treatment on post-test scores was significant for suicidal ideation (P<0.01), estimated at 10.2%. Conclusions: The results suggest that this approach can be helpful in effectively helping to improve body image anxiety, as well as reducing suicidal ideation, in adolescent girls with BDD.