This study investigates the effect of problem-based STEM (Science, Technology, Engineering, Mathematics) activities on 7th-grade students' mathematics achievement, attitude, anxiety, self-efficacy, interest, and views. At the same time, the effects of these activities on students' anxiety, self-efficacy towards mathematics, and interest in STEM occupations were also examined. The study sample, which was selected using the convenience sampling method, consisted of 115 7th grade students of a public middle school in Turkey. In the quantitative phase of the research, a quasi-experimental research model with pre-test and post-test control group was used to determine the students' mathematics achievement, attitudes, anxiety, self-efficacy towards mathematics and interests in STEM careers. In the qualitative phase semi-structured interview was used to understand students' views on problem-based STEM activities and mathematics lessons in the qualitative part. Within the scope of the research, 6 different problem-based STEM activities related to ratio-proportion and percentages were applied to the experimental group students. The data collected from both groups before and after the implementation process were analyzed with descriptive statistics, independent samples t-test, and paired sample t-test. The results showed that problem-based STEM activities affect students' mathematics achievement, self-efficacy, and interest in mathematics. It also helps students reducing their mathematics anxiety.