The goal of this study was to assess the feasibility and effectiveness of a theory-based online intervention designed to improve stress management in undergraduate students. The intervention focused on present control because it has been found to be associated with a range of positive outcomes, including lower levels of depression, anxiety, and stress, controlling for a range of other variables (e.g., Frazier et al., 2011, 2012). Two pilot studies were first conducted to confirm that our intervention could increase present control. We then randomly assigned psychology students (n = 292) who were prescreened to have lower scores on the present control subscale of the Perceived Control Over Stressful Events Scale (Frazier et al., 2011) to 1 of 3 conditions: the present control intervention, the present control intervention plus feedback, and stress-information only. Seventy-six percent (n = 223) began the intervention, and 87% (n = 195) of those completed the posttest and 3-week follow-up. The 2 present control intervention groups had lower levels of stress, depression, and anxiety symptoms (on the Depression Anxiety Stress Scales; Lovibond & Lovibond, 1995) and perceived stress (on the Perceived Stress Scale; Cohen, Kamarck, & Mermelstein, 1983) relative to the stress-information-only group at posttest and 3-week follow-up (mean between group d at follow-up = .35, mean within group d for intervention groups at follow-up = -.46). Further, mediation analyses revealed that these effects were mediated by changes in present control. Our intervention represents a potentially valuable tool for college mental health services.