More knowledge is needed regarding what works to prevent oral health problems and reduce disparities in oral health. Motivational interviewing (MI) has demonstrated utility for improving health behavior, including among those with severe mental illness. This study tests whether MI enhances the efficacy of an oral health education intervention in individuals with severe mental illness. Sixty individuals were randomly assigned to MI plus oral health education or oral health education alone. Plaque scores, oral health knowledge, and self-regulation were assessed at baseline and at 4 and 8 weeks. Repeated-measures ANOVA showed improvement (p < 0.05) in plaque, autonomous regulation, and oral health knowledge across time for both groups; however, individuals receiving MI improved significantly more when compared with those receiving oral health education alone. Results suggest that MI is effective for enhancing short-term oral health behavior change for people with severe mental illness and may be useful for the general population.