Purpose: We conducted a randomized control trial of an instructional program, SRSD Plus, which integrates reading for writing to inform, writing, oral language, spelling and handwriting for students in Grades 1 and 2. Method: A total of 10 teachers and their 248 students in Grade 1 (n = 121) and Grade 2 (n = 127) in the southwestern part of the US participated. The teachers included 6 White individuals, 2 Asian Americans, 1 Hispanic individual, and 1 person from another racial or ethnic background. Among the students, 46% were Asian American, 33% were Hispanic, 14% were White, and 5% were identified as multiracial. Teachers were randomly assigned to the SRSD Plus or business-as-usual (BAU) condition; students were assessed at pretest and posttest on writing, oral language, transcription, and reading skills. Results: Multilevel model results showed that students in the SRSD Plus condition outperformed those in the BAU condition in spelling, vocabulary, sentence proficiency, planning from source text, discourse knowledge, and writing productivity and writing quality in source-based informational genre (0.14 [less than or equal to] Effect Size [ES] [less than or equal to] 0.72), but not in word reading or handwriting fluency. Furthermore, although not the target genre, there was a positive transfer effect on writing outcomes in the opinion genre (0.22 [less than or equal to] ES [less than or equal to] 0.34). Conclusion: Integrated reading and writing instruction in SRSD Plus can improve primary grade students' writing, discourse knowledge, planning, oral language, and spelling skills. Theoretical and practical implications and future directions are discussed in the context of the Science of Teaching Reading.