This research was conducted to determine whether there is any effect of the Montessori approach-based STEM activities on the development of the problem-solving skills of pre-service preschool teachers or not. The mixed-method was employed in the research. The sample of the research consisted of 50 teacher candidates studying at the Faculty of Education Preschool Teaching Department in the 2017-2018 academic year. A single grouped pretest-posttest pattern was created to determine the difference in the problem-solving skills of the pre-service teachers in the research. The quantitative data gathered during the process of the research were analysed with the statistical methods; the qualitative data were analysed with the content analysis. In the study, quantitative data were collected using the "Problem Solving Scale", and qualitative data were collected using the "Semi-Structured Interview Form". The interview form is quantitative and consists of 10 open-ended questions in total. This form was applied only to 15 volunteers at the end of the study. For 14 weeks, candidates were given Montessori approach-based STEM training. In the findings obtained as a result of the research; it was observed that there was a significant difference between the pre-test and post-test scores of problem-solving skills and the qualitative analysis results also supported the quantitative results. With these findings, it was concluded that the activities developed the problem-solving skills of individuals and produced solutions to the problem from different perspectives.