Abstract Backgrounds The suicide mortality rate in Korea has been the highest in the Organization for Economic Co-operation and Development countries between 2003 and 2016. The trends of suicide mortality and its inequalities according to socioeconomic positions provide important information in establishing suicide prevention policies. This study investigated trends in suicide mortality and its income-based inequalities in Korea between 2005 and 2017. Methods Data from 2005-2017 National Health Insurance Database individually linked with cause-of-death data of Statistics Korea were employed. A total of 164,412 suicidal deaths from 574,610,162 subjects aged 10 or over were analyzed. Age-standardized suicide mortality rates were calculated by the calendar year, sex, and income quintiles. The slope index of inequality was calculated as an absolute measure for inequality and relative index of inequality was used as a relative inequality measure. Results The suicide mortality in Korea increased from 29.3 per 100,000 in 2005 to 33.3 in 2011, then decreased to 23.1 in 2017. Suicide mortality rates in men were more than twice as high as the rates in women and the gender gap remained throughout the period. The lower income-level, the higher the suicide mortality rate, especially in men. The absolute inequality in suicide measured by the slope index of inequality has changed as the suicide mortality rate fluctuated, but the relative inequality in suicide has increased since 2012. Conclusions The suicide mortality in Korea has increased due to the global financial crisis and copycat suicide following celebrity suicides in 2008, but it has decreased since the paraquat ban in 2011-12. Despite the recent decline in suicide mortality, the gap between income quintiles did not change meaningfully during the study period. In particular, the magnitude of relative inequality has increased. Progressive national policies should be implemented to reduce suicide mortality and its income gaps. Key messages Despite the recent decline in suicide mortality in Korea, the income-based inequalities in suicide did not change meaningfully. The magnitude of relative inequalities has increased recently.