The goal of this randomized controlled trial (RCT) was to evaluate the effectiveness of a small-group intervention in fractions for fifth-graders who are performing below grade level in mathematics. The impact of the fractions intervention was assessed on fifth-grade at-risk students' understanding of foundational fractions concepts and procedural competence with fractions. For the fractions intervention, lessons from the "TransMath"® curriculum (Level 2; Woodward & Stroh, 2015) were modified to create 52 thirty-five-minute lessons focused only on fourth- and fifth-grade level fractions content that could be used in a small-group setting. In this rigorous large-scale RCT, a sample of 1,123 students from three school districts across two U.S. states were screened at the beginning of the school year using a fractions measure developed by the research team. Two hundred and five students who scored between the 15th and the 37th percentile on the screening measure and received parental consent to participate were randomly assigned to the intervention (n = 102) and comparison (n = 103) conditions. Students in the fractions intervention condition received 35-minute tutoring sessions 3-4 times a week. Fraction instruction using the 52 "TransMath" lessons was provided by trained tutors. The comparison condition (n = 103) was business as usual instruction (i.e., core classroom fractions instruction, including intervention or support traditionally provided by the school). Results from the final analytic sample of 186 students (87 in intervention, 99 in comparison) showed that the intervention group significantly outperformed the comparison group on all outcome measures, which included an array of assessments used to measure both student understanding of foundational fractions concepts as well as procedural competence with grade-level fraction material (Hedges' g = 0.66 to 1.08; p < 0.0001).