A socioeconomic status (SES) gap in mathematical knowledge emerges early and widens prior to school entry. To address this gap, a curricular intervention, "Pre-K Mathematics," was developed and found to be effective in prior efficacy research. In the present project, the next step was taken in evaluating this intervention. Specifically, an effectiveness study was conducted to determine the degree to which the intervention improves pre-kindergarten (4-year-old) children's mathematical knowledge when implemented by local program staff in multiple settings that serve a heterogeneous population of low-SES families. In contrast with the prior efficacy study, the effectiveness study required that all teachers, rather than volunteer teachers, in their public preschool programs be available for random selection and random assignment. It also used curriculum coaches who were either members of the participating school districts or Head Start programs' permanent training staff or independent contractors, depending on the way a program a routinely supported teacher learning for its in-service teaching staff. Participating programs included publicly funded Head Start and state preschool programs serving low-income, ethnically/racially diverse, urban families in California and low-income, predominantly White, rural families in Kentucky and Indiana. A trainer-of-trainers model was used (1) to train curriculum coaches to support teachers' implementation and (2) to train teachers to implement Pre-K Mathematics with adequate fidelity. A two-condition (treatment and control) RCT was conducted, with clusters of pre-kindergarten classrooms as the unit of randomization. Treatment teachers implemented Pre-K Mathematics and control teachers continued their usual classroom practices. Children were assessed at pretest, posttest, and kindergarten follow up using the Child Math Assessment (CMA) and the Test of Early Mathematics Ability, 3rd Edition (TEMA-3). Classroom observations were made to measure the nature and amount of math support provided by treatment and control teachers during the school year. Coaches supported implementation and teachers implemented with adequate to high levels of fidelity. Multi-level analyses revealed that treatment children made significantly greater gains in mathematical knowledge than control children during the pre-kindergarten year as measured by the CMA (ES = 0.83) and TEMA-3 (ES = 0.45). A multilevel mediation analysis found evidence that time spent in mathematically focused small-group activities had a significant indirect effect on children's math outcomes. Thus, this effectiveness study found that the Pre-K Mathematics intervention had a significant positive effect on low-SES children's mathematical knowledge. An implication of this finding is that early mathematics intervention is a promising educational strategy for reducing the SES gap in mathematical knowledge.