The objective of this research is to determine the predictive situation of the proficiency perceptions of preschool teacher candidates' attitudes regarding STEM Education and Problem-Based Learning (PBL) towards 21st century skills. Within the scope of this objective, correlational survey design, one of the quantitative research methods, was used. The sample of the research consisted of a total of 284 teacher candidates studying in the Department of Preschool Education. In order to measure the 21st century skills of the teacher candidates according to their own perspective, the '21st Century Skills and Competences Scale' was used. To measure the attitudes of the teacher candidates towards STEM education, the 'STEM Education Attitude Scale' was used. In addition, the 'Problem-Based Learning Attitude Scale' was employed to identify the candidates' attitudes towards problem-based learning. Standard multiple linear regression analysis was performed while analyzing the data. As a result of the research, it was observed that the attitudes of the preschool teacher candidates regarding STEM education and PBL had a moderate and meaningful relationship with their proficiency perceptions towards 21st century skills. Additionally, it was found out that the candidates' attitudes towards STEM education and PBL explained 20% of their proficiency perceptions towards 21st century skills. In the light of the results obtained from the research, in order for preschool teacher candidates and teachers to design appropriate and qualified learning environments, it is suggested that they can be compared with enriched teaching practices that will be supported with different and innovative approaches, such as STEM and PBL, and structured in the focus of developing 21st century skills.