J-League leaders Sagan sack manager Yoon
Lee Dae Ho of Korea (R) is congratulated by coach Yoon Young Hwan (L) after a home run against Taiwan in Guangzhou on November 19, 2010 - by Nicolas Asfouri
Team officials told a news conference that Yoon's contract had been terminated due to a breakdown in trust between the South Korean and some of his players.
"It was a painful decision," said technical chief Takayuki Nagai. "We are top. But it's a question of what will happen if things turn against us. It is about consideration and care.
"There is no issue with (Yoon's) relationship with players in the first team. It is way of dealing with those who aren't in the side."
Nagai added that talks held with former South Korea international Yoon over a contract extension had broken down during the World Cup.
Yoon, a member of the South Korea squad which reached the 2002 World Cup semi-finals, had guided Kyushu-based Sagan to top spot at the midway point of the J-League season, above Urawa Reds.
The 41-year-old took charge in 2011, leading them from the Japanese second division to the top flight. Assistant coach Megumu Yoshida has replaced him until the end of the season.