This study examined the impact of teaching a planning strategy for planning argument-based written responses on the writing performance of ninth grade students in the context of social studies. The Self-Regulated Strategy Development (SRSD) model was used as an instructional framework to teach the planning strategy. Pre- and posttest writing samples from students who learned the strategy (n = 51) were collected and compared with writing samples from a comparison group (n = 39) who did not learn the strategy. Writing samples were analyzed for overall writing quality, argument-based components, and length. Analysis revealed that students who learned the planning strategy, on average, wrote longer responses and had higher scores on a measure of argument-based writing than students who were not taught the strategy. The results of this study support the use of teaching a planning strategy and SRSD for argument-based writing in social studies.