This study aimed to investigate the effect of curriculum-generated play instruction on the mathematics teaching efficacy of early childhood education pre-service teachers. The study used a one group pre-test/post-test experimental research design, supported by a qualitative approach. The participants of the study consisted of 35 pre-service teachers in the Early Childhood Programme who were registered for a "Play in Early Childhood" course. At the beginning of the study, the "Mathematics Teaching Efficacy Belief Instrument" (MTEBI) was administered. Then the curriculum-generated play instruction, which involved pre-service teachers teaching to their classmates, was applied during the course. At the end of the 16 week study, the MTEBI was again administered. During the study, qualitative data was gathered using open-ended questionnaires and observations. The results showed that the teaching efficacy of the pre-service teachers increased. In addition, the qualitative data supported the quantitative data, showing that the pre-service teachers felt they could teach mathematics more effectively after being taught the play instruction method.