A large body of research supports the efficacy of small group reading interventions for students in Grades K through 3. However, there are few studies evaluating the effects of supplemental Tier-2 intervention implemented within a response to intervention (RTI) or multitiered systems of support (MTSS) framework. The purpose of this study was to evaluate the effects of providing Tier-2 supplemental intervention to students in Grades 1 through 3 identified as experiencing reading difficulties (n = 318) in four elementary schools across four different school districts that were selected to participate in a state MTSS initiative. The supplemental intervention was evaluated using a regression discontinuity design, and results indicated statistically significant overall effects on measures of phonemic awareness and word decoding and no discernable effects on reading fluency and comprehension. Results suggest that supplemental reading intervention implemented within MTSS frameworks can impact key reading outcomes when intervention significantly increases instructional intensity.