We examined effects of a 12-week, teacher-delivered, morphology-infused vocabulary intervention on writing outcomes for 4th and 5th grade U.S. students. The intervention (1) drew attention to the morphological structure of 40 Tier 2 words drawn from the Academic Word List, (2) taught morphologically related forms of those words, and (3) provided opportunities for students to use morphologically complex words in reading and writing. Multilevel model results of posttests showed that intervention students (n = 246) outperformed control students (n = 273) on multiple researcher-developed measures, more accurately judging the meaning of instructed words and manipulating the morphological structure of both instructed and non-instructed words to fit more as well as less constrained sentence contexts. Intervention students also showed significantly larger gains on a standardized spelling assessment, although their gains on a standardized essay test failed to reach traditional significance levels. These results suggest that teacher-delivered morphological instruction can improve the writing of upper elementary students, at least at the word and sentence level, in addition to previously documented positive effects on reading.