Using a randomized control trial, this study examined the causal evidence of cross-language transfer of phonological awareness and letter knowledge (names and sounds) using data from multilingual 1st-grade children (N = 322) in Kenya. Children in the treatment condition received an 8-week instruction on phonological awareness and letter knowledge in Kiswahili. The comparison group received business-as-usual classroom instruction. Children in the treatment condition showed greater improvement in phonological awareness and letter-sound knowledge in Kiswahili and English (positive transfer; effect sizes from 0.37 to 0.95), whereas a negative effect was found in letter-name knowledge (interference; effect size, g = 0.27). No effects were found in reading, nor did the results vary by moderators (e.g., Kiswahili vocabulary). Path analyses revealed divergent patterns of results for different outcomes. Results provide causal evidence for cross-language transfer of phonological awareness and letter knowledge and offer important theoretical and practical implications.