BackgroundSocial isolation and social connectedness are health determinants and aspects of social well‐being with strong associations with psychological distress. This study evaluated relationships among social isolation, social connectedness, and psychological distress (i.e., depression, anxiety) over 1 year in young adult (YA) cancer survivors 18–39 years old.MethodsParticipants were YAs in a large cohort study that completed questionnaires every 2 months for 1 year. Social isolation, aspects of social connectedness (i.e., companionship, emotional support, instrumental support, and informational support), depression, and anxiety were assessed with Patient‐Reported Outcomes Measurement Information System short form measures. Mixed‐effect models were used to evaluate changes over time. Confirmatory factor analysis and multilevel structural equation modeling were used to define social connectedness as a latent construct and determine whether relationships between social isolation and psychological distress were mediated by social connectedness.ResultsParticipants (N = 304) were mean (M) = 33.5 years old (SD = 4.7) and M = 4.5 years (SD = 3.5) post‐initial cancer diagnosis. Most participants were female (67.4%) and non‐Hispanic White (68.4%). Average scores for social well‐being and psychological distress were within normative ranges and did not change (p values >.05). However, large proportions of participants reported at least mild social isolation (27%–30%), depressive symptoms (36%–37%), and symptoms of anxiety (49%–51%) at each time point. Across participants, more social isolation was related to less social connectedness (p values <.001), more depressive symptoms (p < .001), and more symptoms of anxiety (p < .001). Social connectedness mediated the relationship between social isolation and depression (p = .004), but not anxiety (p > .05).ConclusionsSocial isolation and connectedness could be intervention targets for reducing depression among YA cancer survivors.