Depression and loneliness are major threats to health in midlife and older adulthood, carrying substantial mental and physical consequences. Although they frequently co-occur, they are conceptually distinct: depression reflects a broader pattern of psychological distress, whereas loneliness captures the subjective perception of social disconnection. Few studies have simultaneously examined multiple relational dimensions in relation to these outcomes, particularly in non-Western contexts. This study applied network analysis to investigate how diverse features of social relationships—including network size, contact frequency, social engagement, perceived support and strain, and relationship quality—relate to loneliness and depression in South Korean adults aged 40–69 years (N = 502, M = 54.4; 49.4% male). The network revealed that loneliness was most strongly linked to friend network size, marital quality, and friendship quality and was positively associated with friend and spousal strain. Depression was directly connected to loneliness and marital quality, while indirectly shaped by broader relational features, including friend network size and dynamics within friendships and marriages. Centrality analyses highlighted the pivotal role of emotionally meaningful ties—particularly with spouses and friends—in shaping these outcomes. Gender-stratified networks further showed that women’s outcomes were more closely tied to the quality of close relationships, whereas men’s were more strongly linked to broader social participation. Together, these findings underscore the interdependent and multidimensional nature of social relationships in midlife and older adulthood, extend evidence to a non-Western context, and suggest that strengthening intimate ties and opportunities for meaningful engagement may help reduce loneliness and depression.
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