Metacognition is theorized to play a central role in children's mathematics learning. The primary goal of the current study was to provide experimental evidence in support of this role with elementary school students learning about mathematical equivalence. The final sample included 135 children (59 first graders and 76 second graders) who participated in the study across three sessions in their classrooms. They completed a pretest during Session 1, a lesson and posttest during Session 2, and a 2-week delayed retention test during Session 3. For Session 2, children were randomly assigned to receive a lesson on mathematical equivalence with or without integrated metacognitive questions. Relative to the control lesson, children who received the metacognitive lesson demonstrated higher accuracy and higher metacognitive monitoring scores on the posttest and retention test. Further, these benefits sometimes extended to uninstructed items targeting arithmetic and place value. No condition effects were observed for children's metacognitive control skills within any of the topics. These findings suggest a brief metacognitive lesson can improve children's mathematics understanding.