The present study investigated the effectiveness of peer-mediated check-in/check-out (CICO) on the internalizing behaviors of elementary school students. A nonconcurrent multiple-baseline design across participants was utilized to evaluate the intervention's effectiveness for 3 students in 1st and 2nd grade. Two 5th grade students were trained to implement CICO under the supervision of an adult intervention specialist. The peer-mediated CICO procedure was effective for 2 of the 3 participants as evidenced by moderate to large effect sizes; however, all 3 participants were identified as "at-risk" on a universal screener for internalizing problems. The results suggest peer-mediated CICO may be a resource-efficient Tier II strategy to meet the needs of students engaging in internalizing behavior within a multitiered framework of service delivery.