Antwerp, who won the Belgian Cup last month, started the match top of the standings ahead of Union on goal difference but would have been crowned champions due to the tiebreaker rules, even though there is another round of matches next Sunday.
Vincent Janssen’s 14th-minute strike put them ahead in front of a packed stadium anticipating a title party, but a cruel deflection off schoolboy Arthur Vermeeren saw Cameron Puertas equalise in the 80th minute for 10-man Union.
Union, in their second season in the top flight after a 48-year absence, had to play the last half hour down to 10 men after midfielder Senne Lynen was shown a red card for a dangerous tackle.
Antwerp won the last of their four Belgian titles in 1957, but their 66-year drought is nothing in comparison to Union, who were the most successful Belgian side before the outbreak of World War Two with 11 championships but last won the trophy in 1935.
Antwerp will win the league next Sunday with victory at Genk while Union host Club Brugge.
The Belgian league plays a regular season of 34 games, after which the top four teams advance to a playoff round to decide the title.