From the Korea Times:

 The Korea Chamber of Commerce and Industry (KCCI) estimated that the underground economy accounts for about 30 percent of the GDP with the size of the market reaching nearly 250 trillion won.

It said it is imperative for the government to reduce the black market size to that of developed countries, as underground economic activities are negatively affecting the growth and tax policies.

The underground economy, or black market, refers to businesses that are conducted illegally out of the government’s radar to evade taxes through untraceable settlement methods such as money laundering or cash transactions.

The article does not specify what types of activities are contributing to the black market in South Korea.

The article also mentions other countries and the estimated size of their respective black market.

 The chamber said Korea’s underground economy is one of the worst among OECD economies, ranking 43rd out of 145. The underground economy in the United States accounts for about 8.4 percent of its GDP; 10.8 percent for Japan; 13.1 percent for Singapore; 15.6 percent for China; and 16.6 percent for Hong Kong.

These figures are from the Korean Chamber of Commerce and Industry.

According to the Havocscope Illicit Market Countries Index, the black market in South Korea stands at $14.2 billion.