Physical activity and healthy diets are determinants of the health of convicts who are deprived of freedom. However, little research has focused on these two aspects concurrently. The aim of this study is to analyze the relationships between the level of prisoners’ physical activity and their dietary habits, diet quality, and nutritional knowledge. The cross-sectional study was conducted with 226 prisoners. The inclusion criteria included male sex, age 18-65 years, and consent for the research. We excluded the subjects who met any of the following conditions: female sex, unwillingness to participate, and dangerous prisoner status. The KomPAN questionnaire was used to assess the prisoners’ eating habits, diet quality, and level of nutrition knowledge. The “Last 7d” short form of the International Physical Activity Questionnaire (IPAQ) was used to assess the prisoners’ level of physical activity. A significant correlation between the prisoners’ age and their levels of physical activity was found (p = 0.008). Prisoners who were aged 18-29 and over 40 declared a low level of physical activity, whereas those aged 30-40 reported a high or medium level. The level of physical activity of the surveyed prisoners was not significantly correlated with their dietary habits, the quality of their diet, or their nutrition knowledge.