Suicide is the fourth leading cause of death in South Korea, which is the highest rate among the Organization for Economic Cooperation and Development member countries. The World Health Organization worldwide initiative for the prevention of suicide reported that there are at least 20 suicide attempts for every suicide committed. The lifetime prevalence of suicidal ideation, planning, and attempts in South Korea was 15.2%, 3.3%, and 3.2% (single 2.1% and multiple 1.1%), respectively. Suicide attempts showed a significant association with mental disorders, especially major depressive disorder in a Korea-based community study. Brain neuroimaging studies showed prefrontal localized hypofunction and impaired serotonergic responsivity in those attempting suicide, which are also typical findings in patients with depression. Postmortem studies have reported that approximately 60% of suicide victims suffered from major depressive disorder and other mood disorders. Family studies have suggested a genetic linkage between suicide and depression. In conclusion, suicide and suicidal behaviors are significantly associated with depression. Suicide is a medical condition which is treatable and preventable, and suicide rates can be reduced through early diagnosis and treatment for depression. Regular surveys for suicide and depression an essential tool for establishing a national policy for suicide prevention.