This study examined the efficacy of a supplemental, multicomponent adolescent reading intervention for middle school students who scored below proficient on a state literacy assessment. Using a within-school experimental design, the authors randomly assigned 483 students in grades 6–8 to a business-as-usual control condition or to the Strategic Adolescent Reading Intervention (STARI), a supplemental reading program involving instruction to support word-reading skills, fluency, vocabulary, and comprehension, and peer talk to promote reading engagement and comprehension. The authors assessed behavioral engagement by measuring how much of the STARI curricular activities students completed during an academic school year, and collected intervention teachers’ ratings of their students’ reading engagement. STARI students outperformed control students on measures of word recognition (Cohen’s d = 0.20), efficiency of basic reading comprehension (Cohen’s d = 0.21), and morphological awareness (Cohen’s d = 0.18). Reading engagement in its behavioral form, as measured by students’ participation and involvement in the STARI curriculum, mediated the treatment effects on each of these three posttest outcomes. Intervention teachers’ ratings of their students’ emotional and cognitive engagement explained unique variance on reading posttests. Findings from this study support the hypothesis that (a) behavioral engagement fosters struggling adolescents’ reading growth, and (b) teachers’ perceptions of their students’ emotional and cognitive engagement further contribute to reading competence.