As a countermeasure to the increased loneliness induced by the COVID‐19 pandemic‐related university closures, universities provided students with online interaction opportunities. However, whether these opportunities contributed to reducing loneliness during the university closures remains unclear, as previous studies have produced contradictory findings. We conducted a nationwide cross‐sectional survey. Data were collected on demographics, social environment, social support, interactions, health and loneliness from 4949 students from 60 universities across Japan. We used psychological network and Directed Acyclic Graphs (DAGs) to examine the effect of online interactions on loneliness during university closures during COVID‐19. The results showed that the frequency of online interactions with friends did not exert a significant influence on loneliness during university closures. A comparative examination of the DAGs further illuminated that the social environment exhibited fewer pathways for interpersonal interactions and social support during these closure periods. The psychosocial pathways influencing young adults' loneliness show variations contingent on the university's closure status. Notably, the impact of heightened online interactions with friends on loneliness appears to be less pronounced among young adults in the context of university closure.