Under South Korea’s constitution, the president has the authority to declare martial law during war, armed conflict, or other national emergencies.
The last time martial law was declared in the country was in 1979, when the country’s long-time military dictator Park Chung-hee was assassinated in a coup.
A group of military leaders, led by General Chun Doo-hwan, declared martial law in 1980, banning political activities and arresting dissidents.
Hundreds of people died amid a crackdown on protesters before martial law was lifted in 1981.
Martial law has not been invoked since South Korea became a parliamentary democracy in 1987.
Yoon pulled the trigger on Tuesday, saying he was trying to save the country from “anti-state forces”.
But some analysts have described the move as his bid to thwart political opposition.
Yoon has been a lame duck president since the opposition won a landslide in the country’s general election in April this year – his government has not been able to pass the laws it wanted and has been reduced instead to vetoing bills the opposition has proposed.
The president’s approval ratings have hit record lows of 17% this year, as he and his wife Kim Keon-hee have been mired in a spate of scandals.
Additional reporting by Woongbee Lee in Seoul and Frances Mao in London