Because suicide is irreversible, prevention is paramount. For the optimal strategy to reduce lethal means, we sought to investigate age- and sex-associated variations in suicide methods. Data on annual causes of death from 1991 to 2015 in the Republic of Korea were used. Major sociodemographic correlates of the five suicide methods were analyzed by multiple multinominal logistic regression analysis. Among a total of 239,565 suicides from 1991 to 2015, hanging was most common. Gas poisoning sharply increased from 2007 to 2015. The gap between hanging and the second most common method of suicide has increased from 659 in 2004 to 4,433 in 2015. Charcoal burning was most commonly used by males younger than 45 years of age, whereas pesticide was commonly used by both sexes ages 55 years and older. Our results suggest that age- and sex-specific suicide prevention strategies are needed, particularly for gas and pesticide poisoning.