The official AFL website promoted merchandise naming Sam Mitchell the code's best and fairest player, when in fact Nat Fyfe was the real winner.
The mistake - attributed to human error - drew harsh criticism on social media.
The AFL's media manager, Patrick Keane, did not hold back either, describing the image as an "epic website fail".
As soon as the mistake was realised the AFL removed the erroneous post.
Screenshots circulating online show the framed "Grand Sam Mitchell 2015 Brownlow Medalist Signed Guernsey" was selling for A$1,364 ($948, Â£625).
"Widely regarded as the premier contested ball winner in the game with a side-step and delivery to match, Mitchell has continued to develop new dimensions to his game," the deleted post said.
The AFL's Patrick Keane tweeted: "[we] had six Brownlow memorabilia options ready to go 4 when winner named. Human error went live with no result known. [The] AFL apologises without reservation to all fans/supporters who have been misled."
Some angry fans demanded a refund on bets they had placed, as local media reports the post pushed Mitchell's odds from A$11 to $3.
The Australian Football League (AFL) has suffered an "epic fail" after it congratulated the wrong player for winning the prestigious Brownlow Medal.