Introduction and Aim of the Research: Physical fitness in young adults is crucial for maintaining mental and physical health and preventing chronic diseases later in life. The aim of this research was to examine the relationship between body composition, leisure-time physical activity, and performance on the Harvard Step Test as factors that may influence the fitness level of students at the University of Rijeka, as well as to explore whether there are differences in results between male and female participants. Participants and Methods: The study included 80 students from the University of Rijeka, both genders, without chronic diseases or deformities. Data were collected using the short version of the IPAQ questionnaire, the Harvard Step Test, and body composition analysis via bioelectrical impedance using a Tanita scale. Data were processed in Statistica 14.0.0.15 using Spearman's correlation coefficient, Mann-Whitney test, and Student's t-test for large independent samples. Results: Body composition, specifically the percentage of fat and muscle, was not statistically significantly associated with the fitness index. There was no statistically significant difference in the fitness index between male and female students. Additionally, a statistically significant positive correlation was observed between the total number of MET-minutes/week and the fitness index. Although there was no statistically significant difference in IPAQ questionnaire results for walking and vigorous physical activity between male and female students, a significant difference was found in moderate physical activity and overall questionnaire results, with male students being more active in their leisure time. Conclusion: The results of this study provided insight into the levels of physical fitness and physical activity among students at the University of Rijeka, as well as the relationship between body composition and physical activity with fitness levels. The research showed that the majority of participants fell within the range of low to average physical fitness, with a noticeable number of participants in poor physical condition.