Background Depression is a major mental health problem among older adults and associated with physical health deterioration and social isolation. As population aging accelerates, the prevalence of chronic conditions and multimorbidity has increased, thereby increasing the risk of late-life depression. Social relationships are recognized as an important protective factor; however, longitudinal evidence on the long-term effects of chronic conditions and social relationships on depression, particularly across income levels, remains limited.Methods This study analyzed adults aged 65 years or older using data from five waves (2014–2022) of the Korean Longitudinal Study of Aging. Depression was measured using a validated short depression scale and dichotomized based on a standard cutoff. Chronic conditions were categorized as none, one, two, or more physician-diagnosed conditions. Social relationships were classified as high, middle, or low based on the frequency of contact with friends, relatives, or neighbors. Generalized estimating equation models were applied to examine concurrent and two-year lagged associations. Income-stratified analyses were conducted to assess socioeconomic differences.Results Numerous chronic conditions and lower levels of social relationships were associated with a significantly higher risk of depression. Multimorbidity increased the odds of subsequent depression by more than 30% in adjusted two-year lagged models, whereas low levels of social relationships were associated with approximately twice the risk of depression compared with high levels. Interaction effects between chronic conditions and social relationships were observed in selected analyses and did not show consistent patterns. Income-stratified analyses indicated that multimorbidity and low social relationships were associated with depression among low-income older adults, whereas social relationships showed a stronger association with depression than chronic conditions among high-income older adults.Conclusions Chronic conditions and social relationships were independently associated with depression in later life, with their relative importance varying by income level. These findings suggest the need for integrated and income-sensitive approaches that address physical health management and social relationship support to prevent depression among older adults.