The aim of the present study was to assess the efficacy of psychoeducation as compared with cognitive behavioral group therapy in adults with attention deficit hyperactivity disorder (ADHD) who still had significant symptoms and were in pharmacological treatment. This is the first study on psychoeducation in adults with ADHD. Thirty-two individuals were randomized to two treatment conditions: 15 were in the psychoeducation group and 11 were in the cognitive behavioral group therapy. A total of 30 completed treatment, and 26 completed the follow-up assessments. The results indicated that both treatments were associated with statistically significant improvements on inattention, hyperactivity, impulsivity, and self-esteem. The patients in both groups showed a decrease in anxiety symptoms and obtained significantly lower scores in depression. Measures on functional impairment showed statistically significant differences on improved quality of life and on lower global severity as perceived in self-report and assessed by clinician report. Psychoeducation demonstrated to be an effective treatment in reducing ADHD core symptoms.