Courses that teach evidence-based interventions to enhance well-being are a public health tool that could be used to improve mental health in the population. We compared the well-being of six cohorts of adult students before and after they completed one of two massive open online courses: The Science of Well-Being (N = 581; 441; 1,228) and a control course, Introduction to Psychology (N = 677; 480; 1,480). Baseline well-being levels were equivalent across all six samples. Students in both courses increased in their well-being from baseline to follow-up in all three samples (p < .001); however, at follow-up, students in The Science of Well-Being course had higher subjective well-being than the control course (sample 1: r = .18, d = .37, p < .001; sample 2: r = .21, d = .43, p < .001; sample 3: r = .19, d = .38, p < .001). Overall, across three samples, we found that students who completed either of these online psychology courses increased in their well-being--but that students in The Science of Well-Being course showed greater improvement. These findings suggest that large free online courses that teach evidence-based approaches to well-being could positively impact mental health at large scales.