Reform mathematics is an approach to mathematics education, particularly in North America. It is based on principles explained in 1989 by the National Council of Teachers of Mathematics (NCTM). The NCTM document Curriculum and Evaluation Standards for School Mathematics (CESSM) set forth a vision for K–12 (ages 5–18) mathematics education in the United States and Canada. The CESSM recommendations were adopted by many local- and federal-level education agencies during the 1990s. In 2000, the NCTM revised its CESSM with the publication of Principles and Standards for School Mathematics (PSSM). Like those in the first publication, the updated recommendations became the basis for many states' mathematics standards, and the method in textbooks developed by many federally-funded projects. The CESSM de-emphasised manual arithmetic in favor of students developing their own conceptual thinking and problem solving. The PSSM presents a more balanced view, but still has the same emphases.

Mathematics instruction in this style has been labeled standards-based mathematics or reform mathematics.

Mathematics education reform built up momentum in the early 1980s, as educators reacted to the "new math" of the 1960s and 1970s. The work of Piaget and other developmental psychologists had shifted the focus of mathematics educators from mathematics content to how children best learn mathematics. The National Council of Teachers of Mathematics summarized the state of current research with the publication of Curriculum and Evaluation Standards in 1989 and Principles and Standards for School Mathematics in 2000, bringing definition to the reform movement in North America.

Reform mathematics curricula challenge students to make sense of new mathematical ideas through explorations and projects, often in real-world contexts. Reform texts emphasize written and verbal communication, working in cooperative groups, and making connections between concepts and between representations. One of principles of reform mathematics is social equity. In contrast, "traditional" textbooks emphasize procedural mathematics and provide step-by-step examples with skill-building exercises.

Traditional mathematics focuses on teaching algorithms that will lead to the correct answer of a particular problem. Because of this focus on application of algorithms, the student of traditional math must apply the specific method that is being taught. Reform mathematics de-emphasizes this algorithmic dependence. Instead of leading students to find the exact answers to specific problems, reform educators focus students on the overall process which leads to an answer. Students' occasional errors are deemed less important than their understanding of an overall thought process. Research has shown that children make fewer mistakes with calculations and remember algorithms longer when they understand the concepts underlying the methods they use. In general, children in reform classes perform at least as well as children in traditional classes on tests of calculation skill, and perform considerably better on tests of problem solving.

Principles and Standards for School Mathematics was championed by educators, administrators and some mathematicians as raising standards for all students; others criticized it for its prioritizing the understanding of processes over the learning of standard calculation procedures. Parents, educators and some mathematicians opposing reform mathematics complained about students becoming confused and frustrated, claiming that the style of instruction was inefficient and characterized by frequent false starts. Proponents of reform mathematics countered that research showed that correctly-applied reform math curricula taught students basic math skills at least as well as curricula used in traditional programs, and additionally that reform math curricula was a more effective tool for teaching students the underlying concepts. Communities that adopted reform curricula generally saw their students' math scores increase. However, one study found that first-grade students with a below-average aptitude in math responded better to teacher-directed instruction.

During the 1990s, the large-scale adoption of curricula such as Mathland was criticized for partially or entirely abandoning teaching of standard arithmetic methods such as practicing regrouping or finding common denominators. Protests from groups such as Mathematically Correct led to many districts and states abandoning such textbooks. Some states—such as California—revised their mathematics standards to partially or largely repudiate the basic tenets of reform mathematics, and to re-emphasize mastery of standard mathematics facts and methods.

The American Institutes for Research (AIR) reported in 2005 that the NCTM proposals "risk exposing students to unrealistically advanced mathematics content in the early grades." This is in reference to NCTM's recommendation that algebraic concepts, such as understanding patterns and properties like commutativity (2+3=3+2), should be taught as early as first grade.