Meta-analyses indicate that positive psychological interventions are effective at increasing positive affect, as well as reducing anxiety and depression; however, it is unclear how well these effects generalize during periods of high stress. Therefore, the current study tested whether a 2-week online positive psychological intervention delivered during the COVID-19 pandemic, a naturalistic stressor, (1) increased positive affect; (2) improved psychological well-being, optimism, life satisfaction, perceived social support, and loneliness; (3) and reduced negative affect in college students, a group known to have high pandemic distress. Participants (N = 250; 76.9% female) ages 18-45 were recruited from the University of Pittsburgh undergraduate subject pool between September and November of 2020. Participants were randomized to the online positive psychological intervention or active control condition and stratified by trait positive affect, sex, and year in college. Participants in both conditions completed one writing activity every other day for two consecutive weeks. Control participants documented their activities for that day (e.g., meals, going to gym). Intervention participants chose from six positive psychology activities. All outcome variables were assessed pre- and post-intervention by validated questionnaires. Across both conditions, positive and negative affect decreased from pre- to post-intervention. No other psychological factor differed by condition, time, or their interaction. The current null findings are in line with a more recent meta-analysis indicating that positive psychological interventions may have smaller effects on psychological well-being and depressive symptoms than was reported pre-pandemic. Study findings may suggest reduced efficacy of virtual positive psychological interventions under highly stressful circumstances. Supplementary Information: The online version contains supplementary material available at 10.1007/s42761-022-00148-z.