The effects of a mathematical problem-solving intervention on students' problem-solving performance and math achievement were measured in a randomized control trial with 1,059 7th-grade students. The intervention, "Solve It!," is a research-based cognitive strategy instructional intervention that was shown to improve the problem-solving performance of 8th-grade students with and without learning disabilities (LD). The purpose of the present study was to determine whether the effectiveness of the intervention could be replicated with younger students. Forty middle schools in a large urban school district were included in the study, with one 7th-grade math teacher participating at each school (after attrition, n = 34). "Solve It!" was implemented by the teachers in their inclusive math classrooms. Problem-solving performance was assessed using curriculum-based math problem-solving measures, which were administered as a pretest and then monthly over the course of the 8-month intervention. Students who received the intervention (n = 644) embedded in the district curriculum showed a significantly greater rate of growth on the curriculum-based measures than students in the comparison group (n = 415) who received the district curriculum only. Results of the Bayesian analyses indicated that the intervention effect was somewhat stronger for low-achieving students than for average-achieving students. Overall, findings from the present study as well as the previous study with 8th-grade students indicate that the intervention was effective across ability groups and is an appropriate program to use in inclusive classrooms with students of varying math ability.