Objectives. Written benefit finding is known to improve psychological and physical health in a range of patient groups. Here, we tested the efficacy of written benefit finding, delivered online during the Covid-19 pandemic lockdown, on mood and physical symptoms. We also investigated perseverative thinking as a moderator of these effects. Design. A quantitative longitudinal design was employed. Main Outcome Measures. Participants (n=91) completed self-report measures of anxiety, depression, stress and physical symptoms at baseline, and two weeks after being randomised to complete three consecutive days of writing about the positive thoughts and feelings they experienced during the pandemic (written benefit finding) or to unemotively describe the events of the previous day (control). State anxiety was measured immediately before and after writing. Perseverative thinking was measured at baseline. Results. Anxiety and depression symptoms decreased between baseline and the two week follow-up, but did not differ significantly between the two conditions. Perseverative thinking was negatively associated with changes in symptoms of anxiety, depression and stress, but did not moderate any writing effects. There was a significant reduction in state anxiety in the written benefit finding condition. Conclusions. Written benefit finding may be a useful intervention for short-term improvements in wellbeing.