Purpose: To assess, in the context of Primary Health Care (PHC), the effect of a psychological intervention in mental health among caregivers (CGs) of dependent relatives. Design and Methods: Randomized multicenter, controlled clinical trial. The 125 CGs included in the trial were receiving health care in PHC. Inclusion criteria: Identifying oneself as principal CG of a dependent relative with dementia or any other disability, and having performed this task for at least 6 months. CGs were randomized to an intervention group (cognitive-behavioral treatment for managing dysfunctional thoughts about caregiving and training in self-help techniques) or to a control group (care as usual). CG mental health (General Health Questionnaire [GHQ-12]), dysfunctional thoughts about caregiving, quality of life, and burden were measured. Results: The intervention group showed improvement in mental health: A mean reduction in GHQ-12 score of -3.33 points was recorded in the intervention group vs. the control group (95% CI: -5.95 to -0.70; p = 0.01; Cohen d = 0.55). Improvement was also recorded in dysfunctional thoughts about caregiving: (-5.84; 95% CI: -10.60 to -1.09; p = 0.01; Cohen d = 0.62). Among the CGs that completed the initial and final assessments, a mean of 4.77 (SD 2.68) attended a maximum of 8 sessions. Men attended more often (5.00 sessions with SD 2.68) than women (4.70 sessions with SD 2.45; p less than 0.001). Implications: Psychological group intervention in the context of PHC, aimed at the CGs of dependent persons with dementia and other disabilities, has improved mental health condition in CGs.