One potential contributor to student achievement that has garnered recent attention is students' mindset. A recent review of the literature on the effect of a growth mindset intervention on students' academic achievement has shown small effects, but limited research has been conducted with young students. This study investigated whether students who receive a growth mindset intervention in addition to regular vocabulary instruction would make greater gains in vocabulary development. First grade students (N = 61) were randomly assigned to two treatment conditions providing vocabulary instruction, with one condition providing additional instruction in growth mindset, and a business as usual comparison group. The vocabulary intervention yielded moderate effects on comprehension, however, there were no significant differences for vocabulary learning. The mindset intervention had little impact on increasing student persistence. A moderator analysis indicated that word reading abilities influenced vocabulary learning; students who started with lower word-level skills benefited more from instruction.