A two-cohort cluster-randomized trial was conducted to estimate effects of small-group supplemental vocabulary instruction for at-risk kindergarten English learners (ELs). Connections students received explicit instruction in high-frequency decodable root words, and interactive book reading (IBR) students were taught the same words in a storybook reading context. A total of 324 EL students representing 24 home languages and averaging in the 10th percentile in receptive vocabulary completed the study (Connections n = 163 in 75 small groups; IBR n = 161 in 72 IBR small groups). Although small groups in both conditions made significant immediate gains across all measures, Connections students made significantly greater gains in reading vocabulary and decoding (d = 0.64 and 0.45, respectively). At first-grade follow-up, longer-term gains were again greater for Connections students, but with smaller effect sizes (d = 0.29 and 0.27, respectively). Results indicate that explicit Connections instruction features designed to build semantic, orthographic, and phonological connections for word learning were effective for improving proximal reading vocabulary and general decoding; however, increases in root word reading vocabulary did not transfer to general vocabulary knowledge.