Purpose We identified the effect of sedentary behavior on stress, anxiety, and depression among Korean university students. Design and Methods Data were collected from 244 students using self-reported sitting time, the Perceived Stress Scale, the Beck Anxiety Inventory, and the Center for Epidemiological Studies-Depression Scale. Findings Mean sitting time was 7.96 h per day. As sitting hours increased, university students' stress, anxiety, and depression significantly increased despite controlling for sex, economic level, body mass index, underlying disease, and health self-management. Practical Implications Intervention programs that reduce sedentary behavior and improve physical activity and mental health for university students are necessary.