Elementary school children (ages 7-11) struggle to understand mathematical equivalence, a foundational prealgebraic concept. Some manipulations to the learning environment, including well-structured nontraditional arithmetic practice alone, have been shown to improve children's understanding; however, improvements have been modest. The goal of this study was to test an iteratively developed supplemental intervention for second grade that was designed to yield widespread mastery of mathematical equivalence. The intervention included three components beyond nontraditional arithmetic practice: (a) lessons that introduce the equal sign outside of arithmetic contexts, (b) "concreteness fading" exercises, and (c) activities that require children to compare and explain different problem formats and problem-solving strategies. After the development process, a small, randomized experiment was conducted with 142 students across eight second grade classrooms to evaluate the effects of the intervention on children's solving of mathematical equivalence problems, encoding of mathematical equivalence problems, and defining of the equal sign. Classrooms were randomly assigned to the intervention or to nontraditional arithmetic practice alone. Analyses at the classroom level demonstrated that the intervention classrooms performed better than the active control classrooms both in terms of pre-to-post change in understanding of mathematical equivalence (g = 1.87) and accuracy on transfer problems at posttest (g = 2.07). Nonparametric analyses led to the same conclusions. Results suggest that the comprehensive intervention improves children's understanding of mathematical equivalence to levels that surpass equivalent levels of well-structured arithmetic practice alone, as well as business-as-usual benchmarks from previous studies.