This article reports the results from a study investigating the effects of a discipline-specific reading and writing intervention (I3C/PROVE IT!) with fourth and fifth graders. Participants included 237 students with writing difficulties (WD) from an initial pool of 608 upper elementary school students in a larger study. Teachers and students were randomly assigned to I3C/PROVE IT! or business-as-usual conditions and then provided instruction on reading historical documents and writing evidence-based arguments. Findings indicated that over a period of almost 3 months, the historical writing growth trajectories of students with WD in I3C/PROVE IT! classrooms were significantly greater than their peers in business-as-usual classrooms. Significant findings favoring I3C/PROVE IT! students also generalized to domain-general measures. This study provides evidence for the benefits of discipline-specific interventions in social studies for students with WD. Limitations and directions for future research are discussed.