ObjectiveThe aim of the present study was to evaluate the effect of a brief version of the behavioral intervention Acceptance and Commitment Therapy (ACT) on reducing gingivitis and plaque levels after 18 weeks.Materials and methodsOne hundred thirty‐five caries‐active young adults (18–25 years of age), recruited from two public dental clinics, participated in this parallel group randomized control trial (RCT). Participants in the intervention (n = 67) received two ACT sessions in combination with standard information on oral health, and participants allocated to the control group (n = 68) received standard information only. Gingivitis and plaque levels were recorded at baseline and at the 9‐ and 18‐week follow‐ups. The effect of the intervention versus standard information alone was analyzed by intention‐to‐treat and per protocol, applying the General Linear Model (GLM). Exploratory analyses for the intervention and control groups were conducted to evaluate the effect of gender and smoking habits on the gingivitis and plaque outcome. The CONSORT guidelines for RCT were followed.ResultsA significant decrease in gingivitis and plaque levels was observed over time, irrespective of treatment allocation. However, the ACT intervention was not significantly more effective at reducing gingivitis and plaque scores than standard information alone, even though the intervention participants had maintained their improvement to a greater extent. The exploratory analysis revealed that females improved their gingivitis and plaque levels significantly more than the males in the intervention group (p = 0.025 for gingivitis and p = 0.013 for plaque).ConclusionA brief ACT intervention was not proven to be more effective than standard information alone at improving oral health in a sample of young adults with poor oral health. However, ACT seems to have a positive effect on oral health among females. (TRN ISRCTN15009620).