Purpose: The purpose of this study was to test the efficacy of the Teaching Early Literacy and Language (TELL) curriculum package for improving the early literacy and oral language skills of preschoolers from low-income families. Method: In a randomized controlled trial (RCT), TELL was implemented in 30 TELL and 28 business-as-usual classrooms. TELL is a Tier 1 whole-class curriculum package that includes a scope and sequence of instruction, lesson plans and materials, high-quality books, curriculum-based measures (CBMs), and professional development training and coaching. Results: Implementation fidelity was high; however, due to COVID-19 school closures, we did not collect end-of-year data for our third cohort of teachers. Results indicated significant TELL effects on all code-related CBMs and two code-related distal measures as well as receptive and expressive vocabulary CBMs. No significant differences were found on distal measures of oral language/vocabulary or listening comprehension. Conclusions: Results of the present study, paired with findings from an earlier TELL RCT with children with developmental speech and/or language impairments, suggest that children enrolled in TELL classrooms achieve higher outcomes on a variety of code-related and vocabulary measures by the end of their final preschool year.