Objective: The primary aim of this randomized controlled trial, conducted in Minneapolis/St. Paul, Minnesota (2014-2019), was to evaluate the effects of a school-based, school nurse-delivered, secondary obesity prevention intervention to reduce excess weight gain among preadolescent children with obesity or at risk of developing obesity. Methods: Parent/child dyads (n = 132) were randomized to the 9-month Students, Nurses, and Parents Seeking Healthy Options Together (SNAPSHOT) intervention (32.5 contact hours) or newsletter-only control group. Eligible children were 8 to 12 years old, proficient in English, and with a BMI ≥75th percentile, calculated using height/weight reported by a parent, school nurse, or clinician. The primary outcome was child BMI for sex/age z-score (BMIz) at postintervention (12 months) and follow-up (24 months). Results: Among children, 63% were non-White, 51% were male, and 51% with obesity, including 21% with severe obesity. Among families, 59% received economic assistance and 30% reported food insecurity. The mean number of intervention contact hours received was 20 (range: 0-32.5). Among dyads (n = 54) receiving the intervention, parents were very satisfied/satisfied with SNAPSHOT and SNAPSHOT staff, 96% and 100%, respectively, and very likely/likely (97%) to recommend SNAPSHOT to others. Most (70%) children liked the kid group sessions "a lot." In an intent-to-treat analysis, there were no significant between-group differences in child BMIz at 12 [0.04; 95% confidence interval (CI) -0.07 to 0.16] or 24 months (0.06; 95% CI -0.08 to 0.20), with participant retention of 92% and 93%, respectively. Conclusions: The SNAPSHOT intervention was well received, but did not improve BMI in a mostly diverse, low-income preadolescent population. Approaches to intervention delivery that are feasible, maximize accessibility, and optimize clinician and school nurse collaboration warrant consideration. Clinical trial registration identifier NCT02029976.