The 21-year-old embarks on his second temporary move away from the club and will further his development under former United Under-23s lead coach Neil Wood.
The Ammies have enjoyed a positive start to the campaign, and currently sit fourth in the table after winning four of their first six games in League Two.
Galbraith will continue his progress in a new environment whilst being closely monitored and supported by United staff.
The Northern Ireland international made his first-team debut in the Europa League clash with Astana in 2019 and has once again trained regularly with the senior set-up this pre-season.