Suicide rates among older adults in Korea are one of the highest in the world. Although prior research found that poverty is associated with suicide mortality among older adults in Korea, it is unknown whether being a recipient of social assistance may reduce suicide risk among older adults living in poverty. We examined the impact of the public assistance program on suicide-related behaviour (SRB). Data from a nationwide cohort in the Korean National Health Insurance Database, including demographic and medical treatment information, linked to the vital statistics for 2010–19 was used. The entire Korean population born before 1951 was included (n = 7,889,086). Flexible parametric survival model was performed to assess whether the risk of SRB hospitalisation and death differed across a) different levels of socioeconomic status, and b) social assistance status among the poor, using gender-stratified models. Older age, chronic disease, poverty, and being out of the labour market were associated with higher SRB hospitalisation and death for both genders. Among those living in poverty, social assistance recipients had lower rates for SRB death and hospitalisation, compared to the low-income non-recipients. Men who were non-recipients had 0.21 higher hazards ratio (95%CI 0.20 to 0.22) for SRB deaths compared to recipients, and the difference was 0.11 (95%CI 0.10 to 0.11) in women, with similar patterns for SRB hospitalizations. Poverty was associated with higher SRB death and hospitalisation among older adults in Korea. Social assistance targeting the older population may help reduce SRB.