Cognitive psychology research has shown that interleaving, wherein learners practice multiple skills or concepts at once, facilitates learning more than does blocking, wherein learners practice only one skill or concept at a time. Despite the advantage of interleaving over blocking observed across a number of domains, limited attention has been devoted to the effects of interleaving on second language (L2) learning. This study examined the effects of blocking and interleaving on L2 grammar learning. In this study, 115 Japanese learners studied 5 English grammatical structures under 1 of 3 conditions: blocking, interleaving, and increasing (i.e., blocking followed by interleaving). Learning was measured using a grammaticality judgment test administered immediately and 1 week after the treatment. Although interleaving led to the highest number of incorrect responses during training, it was more effective than blocking in the 1-week delayed posttest. These results indicate that the advantage of interleaving extends to L2 grammar learning. Furthermore, learners' levels of prior knowledge were found to moderate the effects of interleaving. Specifically, participants with lower pretest scores benefited more from interleaving compared to those with higher pretest scores. Pedagogically, the findings suggest that grammar learning may be enhanced by incorporating interleaved practice.