Source: Sharecast
Invoking emergency powers, Yoon Suk Yeol accused the country's opposition of controlling parliament and siding with North Korea.
The initial reaction in financial markets included a small pullback on Wall Street, whilst the US dollar jumped by nearly 2% against the South Korean won.
As of 1615 BST however the Greenback had pared its gains to stand 1.22% higher.
Yoon's declaration of martial law came amid attempts to pass a budget bill for the following year.
Social media posts reportedly showed tanks lining the streets.
Shortly after Yoon's announcement, the country's parliament voted to block the move.
Mark Williams, chief Asia economist at Capital Economics, saw a binary outcome from Yoon's declaration, either the suspension of democracy or his ouster.
Williams noted how there was little else to indicate an imminent threat from Pyongang save Yoon's own declaration.
"Assuming the border remains peaceful and beyond the inevitable short-term shock to financial markets, a period of political instability lies ahead in South Korea that will dent confidence in the economy."
-- More to follow --