We examined how high school students use their cognitive and metacognitive skills in the geometry problem-solving process. This research employed a mixed-methods descriptive sequential design. Data were collected in the 2019-2020 academic year at secondary education institutions in the central districts of Adana, Türkiye. Using stratified sampling, 313 students participated in the quantitative component, of 313 students 149 are girls and 164 were boys and they were all 15 years olds. Then, using extreme case sampling, 18 students were selected as participant of the qualitative component. Fourteen of them were boys and four were girls. Measures included the metacognitive skills scale, geometry problem test, thinking-aloud protocol, and an observation form. Descriptive statistics and content analysis were applied for data analysis. Results showed that students with high metacognitive skills used metacognitive skills more when solving geometry problems than students with low metacognitive skills. As the implication of the result it is suggested that attention should be paid to the development of students' metacognitive skills in schools. In this context, it would be beneficial to train teachers to develop metacognitive skills. In addition, there is a need to investigate the effect of metacognitive skills on learning in different learning areas.