Summer is a critical time for students with or at-risk for reading difficulties. This study examines the added benefit of a one-on-one intensive reading intervention presented during a school-based summer literacy program targeting first through fifth-grade students' foundational reading skills. The individually administered, UFLI-Intensive intervention used an explicit, systematic, and intensive approach to support students' reading needs. Pre- and post-test data were available for 62 students attending the summer literacy program. Thirty-seven (n = 37) students had been selected to receive the UFLI-Intensive intervention based on grade-specific oral reading fluency (ORF) cut-scores. We used a regression discontinuity (RD) design to compare reading outcomes of students receiving only the general summer program instruction and those students receiving the UFLI one-on-one intervention. Across all measures assessed, cross-grade results suggested that the intervention showed statistically significant treatment effects on students' decoding skills. These findings demonstrate that the implementation of an intensive, one-on-one intervention can help address the problem of summer reading loss and improve reading outcomes for students with reading difficulties.