A second-half double from Lee Jung-Ho ended Ulsan's run of six consecutive wins as Busan came from a goal down to secure a memorable victory.
Victory for Kim Ho-Gon's side would have secured their first title since 2005 but instead they face a title-deciding match against Pohang Steelers on Sunday.
Santos' 90th minute strike secured the three points for Seo Jung-Won's side, and leaves Jeonbuk and Seoul with a straight fight for the third and final AFC Champions League spot when they meet on Sunday.
In the Relegation Round, Daejeon Citizen's valiant attempt to rescue their season ended in vain as a 1-1 draw with Gyeongnam saw them relegated from the top flight.
The point was enough to secure Gyeongnam's place in the top tier, but Gangwon missed the chance to secure their own safety, drawing 2-2 with second-bottom Daegu.
Choi Seung-In's late double rescued Gangwon from two goals behind, and they will now need a positive result against Jeju United on the final day, although defeat for Daegu at home to Gyeongnam will secure their place in the relegation play-off - in which they will have another chance to preserve their top-flight status against Sangju Sangmu.
Seongnam ended their campaign with a 1-0 victory over Chunnam Dragons, Kim Dong-Sub's strike nine minutes from time was enough to secure all three points for the visitors.
