The publication presents the results of extensive research focused on the relationship between the level of geometric thinking and mathematical competences of students in the 9th grade of primary school, i.e. at the end of lower secondary education in Slovakia. The theoretical basis of the research is van Hiele's theory of the development of geometric thinking and the concept of three levels of mathematical competences according to PISA. The research on a sample of 781 students uses the standardized van Hiele geometric test and the author's Mathematical Competence Test in Geometry. The research also includes the identification of students' misconceptions in the area of ​​geometric knowledge and the analysis of errors they made when solving both tests. The results indicate a connection between the level of geometric thinking of students and their ability to solve complex geometric problems.