We evaluated the impact of a supplemental, small-group kindergarten vocabulary intervention designed to reinforce content taught in core classroom instruction implemented within a multitiered system of support (MTSS) framework. Kindergarten teachers implemented a published vocabulary program with all their students during whole-class instruction for 15 min to 20 min per day over the course of the year. We identified students at risk for language and learning difficulties who scored below the 30th percentile on the Peabody Picture Vocabulary Test-4 (PPVT-4; Dunn & Dunn, 2007) and randomly assigned them in clusters to either the control group (n = 453) that received only the classroom vocabulary instruction, or to the treatment group (n = 468) that received the classroom instruction plus small-group supplemental intervention for 30 min, four times per week between November and May. Analyses using multilevel modeling indicated that students who received supplemental vocabulary intervention outperformed control group students on measures of target vocabulary taught during whole-class and small-group instruction and listening comprehension of passages that included taught vocabulary. There were no effects on standardized measures of vocabulary. At-risk students who received the supplemental intervention also eliminated vocabulary learning differences with typically achieving students (n = 430) who received only classroom vocabulary instruction on words targeted for instruction. Findings suggest that supplemental vocabulary intervention that reinforces content taught during classroom instruction implemented within an MTSS framework can accelerate the learning of at-risk students on proximal and near transfer outcomes that are aligned with the content and focus of the instruction and that MTSS offer a feasible framework for schools to provide effective and efficient vocabulary supports to students in the primary grades. Educational Impact and Implications Statement Vocabulary knowledge, or understanding the meaning of words, is essential for reading success. However, there are significant differences in vocabulary knowledge among many young students who are entering school, and traditional classroom vocabulary instruction does little to narrow these vocabulary differences. Therefore, we evaluated the effects of providing kindergarten students at risk for language and learning difficulties with small-group vocabulary intervention that supplemented general classroom instruction over the course of the school year. In our primary experimental comparison, we found that at-risk students who received the vocabulary intervention experienced accelerated vocabulary learning compared with at-risk students who only participated in classroom vocabulary instruction. In our secondary nonexperimental comparison, we found that supplemental vocabulary intervention was associated with narrowing the vocabulary learning differences between these students and their not-at risk peers. Findings from this study provide guidance to schools and teachers on how to provide more effective vocabulary instruction to young school-aged students.