Objectives: The aim of the present study was to investigate the association between trait gratitude and loneliness in a Dutch population sample of adults over 40 years. In addition, the mediating role of psychological flexibility and engaged living between trait gratitude and loneliness was assessed. Method: A total sample of 163 adults of which 65 men (40%) and 98 women (60%) between 41 and 92 years (M age ¼ 66, SD age ¼ 12) participated in this study. Data from the Loneliness Questionnaire, Flexibility Index Test, Engaged Living Scale and the Short Gratitude, Resentment, and Appreciation Test were used. Mediation analysis was performed. Results: Analysis showed a negative association between trait gratitude and loneliness. In addition, after adjusting for the demographic variables age, gender and educational level, the association between trait gratitude and loneliness was fully mediated by psychological flexibility, and partially mediated by engaged living. Conclusion: This study endorses the importance of trait gratitude and psychological flexibility in relation to experiencing loneliness. Further research is needed to replicate these findings in a more diverse sample and to investigate the causal relationships between these constructs. It would also be interesting to further investigate the role of different age groups and goal (dis)engagement strategies in this relation.