Misconceptions about psychological phenomena are prevalent among students completing college-level psychology courses. Although these myths are often difficult to eliminate, efforts incorporating a refutational focus have demonstrated some initial promise in dispelling these beliefs. In the current quasi-experimental study, four sections of an online undergraduate Abnormal Psychology course (n = 113 total students) were randomly assigned to receive either a myth-debunking poster assignment or class as usual. Students in the myth-debunking sections were assigned one of five mental health-focused myths and corresponding refutational readings to guide their development of posters aimed at informing their classmates about the misconception, disputing the misconception, and citing relevant evidence as support. Beliefs about common misconceptions (five directly addressed in the assignment and five filler myths) were measured at the beginning and end of the semester. Results indicated that students in the myth-debunking condition were significantly (p < 0.001, d = 1.09) more likely to know the truth, at the conclusion of the course, compared to the control group. Overall, the myth-debunking intervention appears to have been effective at reducing students' misconceptions about popular psychological myths, perhaps even some non-targeted psychological misconceptions.