The purpose of the study was to explore the effect of the Montessori Method integrated with cooperative learning on mathematical reasoning skills in the preschool period. In this study, an experimental design with a pretest-posttest control group was used. The study group included a total of 30 children, 15 of whom were in the experimental group and 15 in the control group. The study was conducted with mathematics teaching supported by the Montessori Method integrated with cooperative learning activities in the experimental group and mathematics teaching supported by the Turkish Ministry of National Education (MoNE) Preschool Curriculum (2013) in the control group. The "Evaluation Instrument for the Early Mathematical Reasoning Skills" was used as the data collection tool, and the data were analyzed with the Mann-Whitney U Test in line with the gender and reasoning variables. When the performances of the experimental group students who were taught mathematics supported by the Montessori Method integrated with cooperative learning and the control group students who were taught mathematics supported by the Turkish MoNE Preschool Curriculum (2013) were compared, a significant difference in favor of the experimental group was determined. However, no significant difference was determined in experimental and control group students' reasoning skills according to gender. By integrating the Montessori Method with different methods, materials, and techniques, its effects on developmental areas can be investigated.