Gambling addiction may lead to stressful situations for the gambler and their proximal social environment. 33 close relatives of disordered gamblers volunteered to attend a self-help group; afterwards, they participated in the Mindfulness-Based Stress Reduction (MBSR) program for 8 weeks. The participants’ perceived stress was assessed before and after each treatment condition, and at 1-month, 3-months, and 6-months follow-ups. Participating in the self-help group did not significantly reduce the stress levels of pathological gamblers’ relatives, whereas the MBSR training produced changes of great magnitude (η2 = 0.88), decreasing stress levels from initially moderate scores to low values. Changes in stress levels were maintained at low levels throughout follow-ups, with participants reporting overall strong engagement with the practice of mindfulness. However, a decrease in practice (number of individuals practicing, frequency and time of exercises) was observed at 6-months follow-up, which suggest that including sessions to strengthen practice could be advisable. The MBSR program may be a useful protocol to reduce stress in pathological gamblers’ families.