This quantitative study investigated the extent to which Hispanic newcomer English Language Learners (ELLs) enrolled in grades 4 and 5, who received math instruction in Spanish, improved their math scores in district and state tests as compared to ELLs who received math instruction in English only. The researcher compared 2017-2018 quarterly assessments for those students who received math instruction in Spanish against secondary archival data from the prior three years for students who had received math instruction in English. The primary conceptual framework used to ground this research included the Common Underlying Proficiency Theory postulated by Cummins (1981), which suggests that while every language is different at the surface level, certain commonalities, such as content learning (skills that demand more complex thinking), exist among them. Well-developed, complex thinking skills acquired in one language can translate into positive outcomes when learning a second language (Cummins, 2012). In general, the results of the current study indicate that ELLs who received math instruction in Spanish exhibited higher gains (in that subject) than those instructed in English-only settings. However, when controlling for gender, time in the U.S., grade, and English proficiency, the results were mixed. Specifically, this analysis demonstrates that math content instruction in Spanish improved achievement by 6% for the treated group compared to < 3% for the comparison group. These findings demonstrated that instructional support in ELLs' first language (Spanish) for at least one hour a day during a six-month period contributed to their academic performance in math more than English-only models used in similar settings. While not a complete panacea for the multiple challenges faced by ELLs in U.S. schools, these results may have great implications for practice. That is, school leaders interested in closing academic achievement gaps for their newcomer ELLs may consider adding on-site, dual-language models to their ESL programs.