Yes, and we have a randomized experimental and control study to show this. We did two sessions a week with the book, Forgiveness Is a Choice, for 6 weeks. After the forgiveness sessions, the participants went from clinically depressed to non-depressed. In contrast, those in the drug-treatment program as usual (the control group) went down in depression, but they remained clinically depressed. Here is the reference to that research:
Lin, W.F., Mack, D., Enright, R.D., Krahn, D., & Baskin, T. (2004). Effects of forgiveness therapy on anger, mood, and vulnerability to substance use among inpatient substance-dependent clients. Journal of Consulting and Clinical Psychology, 72(6), 1114-1121.