Therapeutic writing can enhance psychological and physical health. Recent studies have suggested that these kinds of interventions can be effective when delivered online. The present study investigated whether positive emotional writing online can influence psychological and physical health in individuals reporting high levels of negative affectivity, who are most likely to benefit from psychological intervention (N = 72, Mage = 28.5, SDage = 8.7), and further, to investigate the potential moderating role of social inhibition. Participants completed self-report measures of physical symptoms, perceived stress, perceived stress reactivity, depression and generalised anxiety, before completing either i) positive emotional writing, or ii) a non-emotive control writing task on an online portal, for 20 min per day over three consecutive days. State anxiety was measured immediately after each writing session, and self-report questionnaires were again administered four weeks post-writing. Socially inhibited individuals exhibited significant reductions in depression and perceived stress reactivity four weeks following positive emotional writing, relative to writing about a neutral topic. The present study supports the efficacy of online therapeutic writing in individuals who, due to their socially inhibited nature, are most likely to benefit from online interventions which avoid interaction with a therapist or other clients.