This randomized controlled trial assessed the efficacy of a five-session version of Skills Training in Affective and Interpersonal Regulation (STAIR) among veterans obtaining treatment in primary care. Veterans who screened positive for either posttraumatic stress disorder (PTSD) or depression (N = 26) were enrolled and randomized into either five-session STAIR or treatment as usual (TAU). Assessments of PTSD symptoms (PTSD Checklist for DSM-5; PCL-5), depression (Beck Depression Inventory-II; BDI-II), emotion regulation (Difficulties in Emotion Regulation Scale; DERS), and social engagement difficulties (World Health Organization Disability Assessment 2.0; WHODAS-2) were assessed at pretreatment, posttreatment, and 3-month follow-up assessments. Participants assigned to the five-session STAIR condition reported significant improvements on all measures, whereas those assigned to TAU showed no change. Group × Treatment interactions were significant for all outcomes, and effect sizes for the interactions ranged from moderate to large, Hedge's gs = 0.81 for the PCL-5, 1.15 for the BDI-II, 0.75 for the DERS, and 0.81 for the WHODAS-2. The results indicate that five-session STAIR, a brief, skills-focused treatment, may be effective in reducing a range of symptoms and in improving social functioning among veterans treated in primary care settings.