OBJECTIVE Suicidal ideation is an important phase in the suicidal process, preceding suicide attempts and completed suicide. Weak social ties and low support from friends or relatives have been significantly associated with suicidal ideation. This study investigated the relationship between social support and suicidal ideation among young and middle-aged adults in Korea. METHODS The Seoul Citizens Health and Social Indicators Survey conducted face-to-face interviews with 10,922 self-reporting adults. Questions were asked to assess suicidal ideation, and several questions focused on social support, social networks, health behaviors, and health status. RESULTS The strongest association in middle-aged adults was that between suicidal ideation and lack of social support. Poor emotional support significantly influenced suicidal ideation in middle-aged men, whereas lack of instrumental support significantly affected suicidal thoughts in middle-aged women, after controlling for sociodemographic factors, health behaviors, and health status. High alcohol use, functional limitations, and stress were related to suicidal thoughts in young adults, whereas depressive feelings had the strongest association with suicidal ideation in middle-aged women. CONCLUSIONS Social support is a crucial independent correlate of suicidal ideation, especially in middle-aged adults in an urban community setting. This study shows that it is essential to provide gender-specific social support to prevent suicide.