Despite the robust relationship between well-being and social relationships, the latter has received little examination within positive psychology activities (PPAs). This study aimed to test whether kindness- and gratitude-based PPAs, through positive social interaction with peers, enhanced relationship satisfaction. Using a longitudinal randomised controlled design, 225 participants were assigned to one of three conditions (relationship-focused, self-focused or control) and completed measures of relationship satisfaction, social support and happiness on three occasions (baseline, post-intervention and six weeks). The experimental PPAs were relationship-focused (involving social interaction) or self-focused (no social interaction). Those who completed relationship-focused PPAs had greater increases in relationship satisfaction than the self-focused and active control activities at six-week follow-up. Additionally, only those in the relationship-focused condition felt their existing friendships had improved at intervention cessation. Regardless of participants’ initial levels of social support, the intervention effects remained. In conclusion, PPAs fostering social kindness and gratitude significantly strengthened relationship satisfaction.