Writing is a critical skill for success in all areas of life, but it is one of the least taught skills in school. Teachers consistently report being unprepared to teach writing. In this study, set in a Southern U.S. boomtown, teachers received two days of practice-based professional development for a ten-week implementation of self-regulated strategy development (SRSD), an evidence-based writing intervention, to support student persuasive and informational writing as well as performance on the state standardized writing exam. This multi-site cluster randomized controlled study evaluated the effectiveness of SRSD on student writing outcomes including prompt adherence, elements, and holistic quality. Multilevel modeling analysis was used to evaluate data from 418 fourth-grade students (256 treatment, 162 control) nested across 33 classes (n = 17 treatment taught by 8 departmentalized teachers; 16 control, 9 departmentalized teachers) within 11 schools randomly assigned to condition. Teachers implemented SRSD with high fidelity (M = 92%; range 91-100%). SRSD had a statistically significant and large effect on prompt adherence (p < 0.001; Hedges' g = 1.87), elements (p < 0.001; Hedges' g = 0.84) and holistic scores (p < 0.001; Hedges' g = 0.87), while holding gender and pretest scores constant. Effects of SRSD on all writing measures were not significantly moderated by students' gender, students' pretest scores, or schools' pretest scores. There were complications with teacher observations, especially related to technology. Limitations and future directions are discussed.