In what was effectively a head-to-head play-off for the K-League championship on Sunday, Pohang went to Ulsan knowing anything less than a win would see the home side claim the title.
Ulsan's home crowd had already started to celebrate their third league title with around five minutes to go at Munsu Cup Stadium but in the 89th minute Kim Won-Il bundled the ball into the net to steal victory and the championship for the Steelers.
Pohang, who last won the K-League in 2007, clinched the double in South Korea after beating Jeonbuk Motors in the FA Cup final last month.
A free-kick with a minute left decided the championship with the Ulsan goalkeeper initially saving but the Steelers kept the ball in play before Kim swung his boot at the ball to send it into the corner and spark pandemonium amongst the visiting fans.
The win - Pohang's fifth in a row in the K-League Classic's championship round - took the Steelers to 74 points, one ahead of Ulsan, while Jeonbuk claimed third and a spot in the AFC Champions League after a 1-1 draw against fourth-placed Seoul.
In the other game on Sunday, Incheon United defeated Suwon Bluewings 2-1 thanks to Lee Hyo-Gyun's 95th-minute winner.
In the relegation round, Daegu were confirmed to go down to the second tier of South Korean football after a 0-0 draw with Gyeongnam.
Daegu will join Daejeon Citizen in the K-League Challenge next season after both teams finished on 32 points following the latter's 2-0 win over Chunnam Dragons on Saturday.
Gangwon may join Daegu and Daejeon in the second division next season after their 3-0 win over Jeju United was not enough to avoid finishing in the relegation play-off position.
Sangju Sangmu will face Gangwon in a relegation-promotion play-off for a spot in the K-League Classic.
