The purpose of this research is to examine the effects of engineering design-based activities carried out in the sixth-grade Matter and Heat unit on the problem-solving skills of students, and the changes in the designing skills of students during these activities. This research, conducted by following the one-group pre-test -- post-test quasi-experimental design, was carried out with 23 students enrolled in a public school. Within the scope of the research, activities structured in line with the Matter and Heat unit learning outcomes were carried out for five weeks. The effect of the activities on the problem-solving skills of students was evaluated with the 'problem-solving skills test'. During the activities, 'worksheets' and 'design products' were used to evaluate the designing skills of students. The data obtained from the problem-solving skills test were analyzed using the Wilcoxon signed-rank test. The engineering design process rubric was used to evaluate the changes in students' design skills during the engineering design-based activities. The results showed that there is a statistically significant difference between students' pre-test and post-test scores in the problem-solving skills test. It has been determined that engineering design-based activities carried out within the framework of the real-world problems contribute to the increase of engineering design skills of middle school students.