This study assessed the effects of a psychological group intervention based on positive psychology in women with breast cancer. 175 women were randomly assigned either to an experimental group, receiving the 14-session intervention (n = 87), or to a wait list group (n = 88) that did not receive any type of intervention. For treatment, a group intervention was applied, based on improving psychological strengths and enhancing positive psychology-based styles of coping. Strength-related outcomes, self-esteem, well-being, and happiness were assessed before and after the intervention. The experimental group showed higher scores on all of the study variables after the intervention. Participants reported improved self-esteem, emotional intelligence-related abilities, resilience, and optimism, as well as positive affectivity, well-being, and happiness. The results show a beneficial effect of this psychological intervention based on positive psychology on female breast cancer patients' psychological health.