There is increasing scientific interest in the potential association between mindfulness and romantic relationship wellbeing. To date, however, experimental studies using active control groups and testing dyadic effects (i.e. examining both actor and partner effects) are lacking. In the current study, romantically involved individuals engaged for 2 weeks daily in either guided mindfulness exercises, or guided relaxation exercises. Participants, and their partners, completed measures of relationship wellbeing at pre- and post-intervention, and at 1-month follow up. The mindfulness intervention significantly promoted relationship wellbeing, for both participants (i.e. actor effects) and their partners (i.e. partner effects). However, these findings did not significantly differ from changes in relationship wellbeing in the relaxation condition. Theoretical implications of these findings for understanding the association between mindfulness and romantic relationship wellbeing are discussed. Moreover, the findings are discussed in light of recent debates about the relative lack of proper control groups in mindfulness research.