Objective: We conducted a randomized controlled trial to determine whether IRISS (Intervention for those Recently Informed of their Seropositive Status), a positive affect skills intervention, improved positive emotion, psychological health, physical health, and health behaviors in people newly diagnosed with HIV. Method: One-hundred and fifty-nine participants who had received an HIV diagnosis in the past 3 months were randomized to a 5-session, in-person, individually delivered positive affect skills intervention or an attention-matched control condition. Results: For the primary outcome of past-day positive affect, the group difference in change from baseline over time did not reach statistical significance (p = .12, d = .30). Planned secondary analyses within assessment point showed that the intervention led to higher levels of past-day positive affect at 5, 10, and 15 months postdiagnosis compared with an attention control. For antidepressant use, the between group difference in change from baseline was statistically significant (p = .006, d = -.78 baseline to 15 months) and the difference in change over time for intrusive and avoidant thoughts related to HIV was also statistically significant (p = .048, d = .29). Contrary to findings for most health behavior interventions in which effects wane over the follow up period, effect sizes in IRISS seemed to increase over time for most outcomes. Conclusions: This comparatively brief positive affect skills intervention achieved modest improvements in psychological health, and may have the potential to support adjustment to a new HIV diagnosis.