Research on the topic of children's mental health and the connection with physical activity is rare in Croatia, and there is not much information about it. Since mental health is mentioned more often in children, research was conducted with upper grades of primary schools. The aim of this research was to check whether physical activity affects mental health (internalized symptoms) in upper elementary school students. In the research participated 212 students (47.2% boys and 52.8% girls) from the fifth to the eighth grade of primary schools in Vinkovci (Vukovar-Srijem County) and Špišić Bukovica (Virovitica-Podravina County). The lowest response was from the fifth grade (16.9%), followed by the 6th grade (25.5%) and the 8th grade (25.5%), the most respondents were from the seventh grade (32.1%). The age range was between 10 and 15 years with an average age of M=12.33 (SD=1.25). During Teacher's Hour, students filled out an online questionnaire consisting of the Children's Anxiety Scale (SKAN; Purić, 1992), the Children's Depression Scale (CDI; Kovacs, 1992) and the Physical Activity Assessment Questionnaire (PAQ-C; Kowalski et al., 2004). Post hoc analysis with Bonferroni's correction showed that students who were insufficiently physically active, were the ones with the most symptoms of anxiety. The most active students showed minor anxiety symptoms. The research showed that engaging in physical activity does not affect the reduction of depression symptoms, considering the devastating results in which 67.5% respondents’ self-assess clinically elevated symptoms of depression, but results did not show age or gender differences. Almost a quarter of respondents, 27.8% assess clinically elevated anxiety symptoms. Furthermore, a statistically significant difference was obtained in the average physical activity during the week and in total physical activity, where boys were more active than girls were, but there is no such difference between the sexes regarding physical activity in free time. However, girls were more anxious in all grades.