Background Sleeve gastrectomy (SG) is effective in the treatment of cardiometabolic complications of obesity but is associated with bone loss. Purpose To determine the long-term effects of SG on vertebral bone strength, density, and bone marrow adipose tissue (BMAT) in adolescents and young adults with obesity. Materials and Methods This 2-year prospective nonrandomized longitudinal study enrolled adolescents and young adults with obesity who underwent either SG (SG group) or dietary and exercise counseling without surgery (control group) at an academic medical center from 2015 to 2020. Participants underwent quantitative CT of the lumbar spine (L1 and L2 levels) to assess bone density and strength, proton MR spectroscopy to assess BMAT (L1 and L2 levels), and MRI of the abdomen and thigh to assess body composition. Student t and Wilcoxon signed-rank tests were used to compare 24-month changes between and within groups. Regression analysis was performed to evaluate associations between body composition, vertebral bone density, strength, and BMAT. Results A total of 25 participants underwent SG (mean age, 18 years ± 2 [SD], 20 female), and 29 underwent dietary and exercise counseling without surgery (mean age, 18 years ± 3, 21 female). Body mass index (BMI) decreased by a mean of 11.9 kg/m2 ± 5.21 [SD] after 24 months in the SG group (P < .001), while it increased in the control group (mean increase, 1.49 kg/m2 ± 3.10; P = .02). Mean bone strength of the lumbar spine decreased after surgery compared with that in control subjects (mean decrease, -728 N ± 691 vs -7.24 N ± 775; P < .001). BMAT of the lumbar spine increased after SG (mean lipid-to-water ratio increase, 0.10 ± 0.13; P = .001). Changes in vertebral density and strength correlated positively with changes in BMI and body composition (R = 0.34 to R = 0.65, P = .02 to P < .001) and inversely with vertebral BMAT (R = -0.33 to R = -0.47, P = .03 to P = .001). Conclusion SG in adolescents and young adults reduced vertebral bone strength and density and increased BMAT compared with those in control participants. Clinical trial registration no. NCT02557438 © RSNA, 2023 See also the editorial by Link and Schafer in this issue.