Background: Transgender men often experience body image dissatisfaction because of incongruence between their gender identity and physical appearance. Masculinizing mammoplasty (MM) aligns physical appearance with gender identity; however, its impact on body image satisfaction in Brazil has not been comprehensively assessed using validated tools. Objectives: To evaluate satisfaction with chest appearance, nipple aesthetics, and body investment among transgender men in Brazil, comparing those who have undergone MM with those who have not, using validated tools, such as the BODY-Q Chest, BODY-Q Nipple, and Body Investment Scale. Methods: This cross-sectional study included 90 transgender men aged ≥18 years recruited between June and September 2024. Participants were allocated to 2 groups: those who had undergone MM (n = 45) and those eligible but had not yet undergone surgery (n = 45). Inverse probability of treatment weighting and regression models adjusted for age, BMI, and education level were used. Results: Participants had a mean age of 32.2 years (standard deviation [SD] ±8.1, range, 19-62) and BMI of 27.9 kg/m2 (SD ±4.8). The MM group had a mean time since surgery of 40 months (SD ±29.2). After adjustment, MM was associated with significantly greater satisfaction with chest appearance (average treatment effect [ATE], 60.98; 95% CI, 53.02-68.93), nipple aesthetics (ATE, 50.61; 95% CI, 38.99-62.23), and body investment (ATE, 11.02; 95% CI, 5.66-16.38). Chest binding was significantly reduced in the MM group (P < .001). Conclusions: Transgender men in Brazil who underwent MM reported higher body image satisfaction and quality of life, supporting the role of this procedure in enhancing mental health. Level of Evidence: 3 (Therapeutic):