The 78-year-old was first found guilty of the buggery charge in August 2019 but Victoria's Court of Appeal ordered a retrial in 2021.
He was found guilty a second time in October 2022.
The man successfully lodged a second appeal, with the court on Friday ruling he should be acquitted.
It was alleged the former scout leader sexually assaulted a 10-year-old boy seven or eight times at a tennis club in regional Victoria between 1978 and 1979.
The alleged victim told the jury he was abused in the club's equipment shed and described in detail that it was a cream weatherboard building with a pitched roof and no windows.
A couple who used the tennis courts in the late 1970s disputed the shed existed during their evidence at trial.
They said they helped build a tin shed at the site because the club needed somewhere to store tennis equipment closer to the courts.
There was no shed at the site before the tin structure was built, and the only cream weatherboard building was a dairy about 500m from the tennis courts, the couple said.
Photos of the site from 2016 also showed a cream weatherboard structure surrounded by grass paddocks and near a brick cow shed.
Court of Appeal Justices Stephen McLeish, Lesley Taylor and Rowena Orr on Friday found the jury should have had reasonable doubt over the man's guilt because of the conflicting evidence.
In reasons published separately, Justice Taylor noted the alleged victim's detailed evidence about the location of his abuse could not have been considered by the jury as a "mere trifling mistake".
"That mistake was so significant and his reliability so damaged that the jury ought to have had a reasonable doubt about his evidence of the (abuse)," Justice Taylor's judgment stated.
"The offending he described could not have occurred."
The three judges ordered the man's guilty verdict be set aside and a finding of not guilty issued in its place.
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