Purpose: Converging evidence demonstrates that robust academic vocabulary and morphology instruction improves literacy outcomes of multilingual adolescents. However, few interventions have focused on teaching word analysis using bound Latin roots, the major meaning-carrying constituents of academic words (e.g. voc meaning "speak" in "advocate"). Moreover, how students leverage a home language (L1; e.g. voz = "voice" in Spanish) to analyze academic words and develop robust representations has been understudied. This contribution to the special issue on the "science of teaching reading" presents results from an intervention focused on high-impact, general academic words, cross-linguistic connections, and inferring unfamiliar word meanings through morphological analysis. Method: We evaluated a 16-week intervention, employing a quasi-experimental design with 192 multilingual middle schoolers (mean grade = 7.7; 63.02% Latino, 20.83% Asian/Pacific Islander, 8.86% Black, and 7.29% unknown). Adjusting for clustering of students (n = 100 intervention) within classrooms, we estimated treatment effects on root meanings, word meanings, orthographic processing, multidimensional vocabulary knowledge, morphological analysis, and reading comprehension. We examined interactions by L1 (Spanish or other) for each outcome. Results: Significant and practically meaningful main treatment effects were found for five of six outcomes (d = 0.98 to d = 0.32), except reading comprehension where effects were positive but small. Interactions showed small advantages for Spanish L1 adolescents for root meanings, word meanings, and multidimensional vocabulary knowledge. Conclusion: Our findings contribute evidence to the science of teaching reading, demonstrating how emphasis on cross-language analysis of morphological relationships and Latin roots supports flexible and generative vocabulary learning for multilingual adolescents.