The authors conducted a cluster-randomized trial to examine the effectiveness of structured teacher adaptations to the implementation of an evidence-based summer literacy program that provided students with (a) books matched to their reading level and interests and (b) teacher scaffolding for summer reading in the form of end-of-year comprehension lessons and materials sent to students’ homes in the summer months. In this study, 27 high-poverty elementary schools (75–100% eligibility for free or reduced-price lunch) were matched by prior reading achievement and poverty level and randomly assigned to one of two implementation conditions: a core treatment condition that directly replicated implementation procedures used in previous experiments, or a core treatment with structured teacher adaptations condition. In the adaptations condition, teachers were organized into grade-level teams around a practical improvement goal and given structured opportunities to use their knowledge, experience, and local data to extend or modify program components for their students and local contexts. Students in the adaptations condition performed 0.12 standard deviation higher on a reading comprehension posttest than students in the core treatment. An implementation analysis suggests that fidelity to core program components was high in both conditions and that teachers in the adaptations condition primarily made changes that extended or modified program procedures and activities in acceptable ways. Adaptations primarily served to increase the level of family engagement and student engagement with summer books. These results suggest that structured teacher adaptations may enhance rather than diminish the effectiveness of an evidence-based summer literacy program.