The judgment, handed down by the Court of Appeal in Melbourne on Thursday, means the Pitch Perfect actress will receive about $3.9 million less than she had originally been awarded.
During the trial, Wilson said her agents had advised her to stop mentioning her age - which was reported to be 29 in 2015 - because "Hollywood is very ageist, especially towards women".
Speaking on behalf of Bauer Media, General Counsel, Adrian Goss, said in a statement: "It was important for us to revisit the award of damages".
But the judge accepted Bauer's argument that there was insufficient evidence Wilson lost millions in potential earnings as result of several defamatory articles in Australian magazines.
"In the lead up to today, major media organisations united in an unprecedented way to support Bauer's appeal in relation to that issue".
It also reduced Wilson's non-economic losses by 50,000 Australian dollars to 600,000 Australian dollars.
"The Court of Appeal in Australia will be handing down their decision in the morning re my defamation case against @bauermedia".
She took to social media on Wednesday night to say regardless of the outcome she had "already won the case".
With defamation payouts capped in Australia at $350,000, University of Melbourne Law Professor Andrew Kenyon told 9news.com.au when news broke of the damages ruling this payout is "way higher" than Australia has ever seen.
The Court of Appeal is yet to determine costs.
Wilson said in April that she and her legal team were "very confident" the original defamation payout would be upheld.