We investigated the subjective experience of loneliness during COVID-19 by analyzing social media postings from March 2020 to January 2021. We collected text data from loneliness-related subgroups of Reddit and sampled 12787 posts that were written in ten consecutive days from each month. The results suggest that when individuals express their loneliness, they show an internal focus of attention on their emotions, desires, and cognitive appraisals rather than an external focus of attention on situations or other people. Linguistic markers of emotions expressed by lonely individuals included depression, anxiety, anger, hate, helplessness, and sadness. Also, loneliness-related topics were generally about their internal states pertinent to various social relationships, interpersonal interaction deficits, and their own lives in broad time perspectives. COVID-19 related loneliness was associated with negative appraisal of one’s situation and reaching out for new relationships online.