McGowan had joined the Doonhammers in the summer but failed to command a regular place.
29-year-old McGowan's previous clubs include Stranraer, Albion Rovers, Oxford United, Clydebank, K A Iceland and Airdrie United.
United's top scorer, Alex Williams has signed a two year extension to his Ayr contract. The 26-year-old Williams, previously of Stirling, Morton, Queen, Clyde and Ross County, has been superb this season in the Ayr United forward line.
Meanwhile Ayr United have also signed Hamilton Academical striker Paul McLeod on a loan deal for the rest of the season.
The 20-year-old has featured in 10 matches for the First Division leaders this term, netting two goals.
McLeod will go straight into the squad for Saturday's home clash with Queen's Park.