Gratitude has been found to have many positive benefits, whether it is introspective or interpersonal in nature. This research explored the differential effects of an interpersonal and intrapersonal gratitude intervention on subjective well-being (SWB). Participants were assigned to one of three intervention conditions that were characterized by a weekly writing task—an interpersonal gratitude letter (n = 73), an intrapersonal gratitude journal (n = 65), or a learning journal (n = 67), which served as the control. A four-week, repeated gratitude intervention design was conducted, wherein participants' SWB was assessed across 12-time points, including a pre- and post-intervention SWB battery to assess the intervention’s overall impact. Participants in both gratitude conditions reported an overall increase in positive affect, supported by text analysis. However, participants who wrote gratitude letters had significantly less negative affect compared to the gratitude journal participants. Further analysis revealed a significant difference in SBW between the two gratitude conditions. Specifically, participants who experienced the intrapersonal gratitude journal-writing task reported a significant improvement in life satisfaction, while participants in the interpersonal gratitude letter-writing task evidenced a significant improvement in perceived social support. The control condition unexpectedly exhibited an increase in SWB that was likely due to the salience of the participants’ scholastic accomplishments. Finally, individual differences, including The Big Five, predicted gratitude and positive affect, consistent with prior research. / M.S. / Gratitude has been found in prior research to have many positive benefits, whether it is introspective or interpersonal in nature. However, not as much research has been conducted to understand if gratitude expressed introspectively or interpersonally is psychologically different in relation to one’s wellbeing. This research therefore explored the psychological benefits—and differences— between intrapersonal and interpersonal gratitude using a gratitude intervention over four consecutive weeks. Participants were either assigned a thank-you letter (interpersonal condition), an introspective gratitude journal (intrapersonal condition), or a learning journal (control condition). Results demonstrated that those who completed the gratitude letter felt more socially supported, while those who completed the gratitude journal saw an improvement in life satisfaction and how grateful they felt. The control group also found an improvement in their life satisfaction, which may have also been due to the introspective nature of their writing task. Feelings of positivity were greater in both gratitude groups compared to those in the control group. The study provided evidence for the psychological differences between introspective and interpersonal gratitude, and the positive role they both serve.