This study aimed to evaluate the effectiveness of the inquiry-based learning method supported by metacognitive strategies on students' problem-solving and problem-posing skills. The research was conducted in two stages using an exploratory sequential design, which is one of the mixed methods research. The case study design was used in the first stage, while the quasi-experimental design with the pretest and posttest for control group was employed in the second stage. The method that is considered effective in students' gaining problem-solving and problem-posing skills was determined in the first stage. In the second stage, the designated method supported by metacognitive strategies was tested with 63 fourth-grade students. The data for the first stage of the study were collected through interviews held with 12 primary school teachers, the mathematics curriculum, and a group of 10 experts' opinions on the methods in the relevant literature. A content analysis and Lawshe's method were employed to analyze the data at hand. Data in the second stage were collected using the problem-solving skills test and problem-posing skills test. The Mann-Whitney U test was used in the analysis of unrelated measurements, whereas the Wilcoxon signed ranks test was employed in the analysis of related measurements. The analysis results in the first stage revealed that the inquiry-based learning method might be effective in improving students' problem-solving and problem-solving skills. Thus, it was decided to utilize this method in the second stage. The results of the analysis in the second stage showed that the inquiry-based learning method supported by metacognitive strategies and the inquiry-based learning method could be effective in developing students' problem solving and problem-posing skills. Moreover, it was concluded that the methods applied were more effective in developing problem-solving skills and structured problem-posing skills of the students from subdimensions compared to the control group, but not effective in the development of semi-structured and free problem-posing skills from sub-dimensions.