The 28-year-old has agreed a one-year contract with Hamilton.
McDonald left Aberdeen when his contract expired earlier this summer.
“I think he can bring a presence and goals to our team. Gary hitting the box can be a big asset for us,” Hamilton boss Billy Reid told the club’s website.
“I felt we needed another addition in the centre of the park, with the loss of James Wesolowski and James McArthur from last season, and we will be missing Alex Neil at the start of the season,” Reid added.
“Gary is the first real attacking midfielder I’ve had since Stuart Taylor in our First Division winning season.
“By his own admission I don’t think he did as well as he could have at Aberdeen last season, but I know that he is raring to go just now. I’m pleased to add him to the squad.”
