Antwerp drew 2-2 at Racing Genk to finish one point ahead of Genk and Union Saint Gilloise, who were on course for a first title in 88 years but threw it away in a stunning conclusion to their last match at home to outgoing champions Club Brugge.
Antwerp looked out of the race with a minute left in their game against Genk as they were trailing 2-1 while Union were 1-0 up at home over Club Brugge but a wild turn of events in both of the last championship play-off matches turned the title race on its head.
As long as Union were winning and Antwerp were behind, the title was headed to the Brussels-based club, who last won the championship in 1935 and only returned to the top-flight last season after a 48-year absence.
But Union’s hopes ended when Japanese striker Shon Homma (22) equalised for an under-strength Brugge in the 89th minute, which suddenly thrust Genk into pole position.
Union went on to concede two more goals in stoppage time and lose 3-1 in a most dramatic reversal of fortunes.
Genk, who only had an outside chance of the title before play began, had to hold onto their lead to win the title but Alderweireld’s powerful shot from just inside the area in the fifth minute of added time handed it to Antwerp instead.
They finished on 47 points and will compete in the Champions League - their first European Cup campaign since losing to Real Madrid in the first round in the 1957-58 competition.
Genk finished second ahead of Union with both on 46 points.
Antwerp, coached by former Dutch international Marc van Bommel, could have won the title last weekend, needing victory at home in their penultimate game with Union but conceded a late equaliser to let the chance slip in front of stunned supporters.
“I have no words for this,” a visibly emotional Alderweireld told Belgian television after his heroics. “Everyone wrote us off, we fought so hard for this. Everyone was against us, but we did it today.”