The former Scotland international, who made 14 appearances for the Red Devils between 2000 and 2005, was a free agent after leaving Turkish club Genclerbirligi earlier this year.
Stewart, 30, recently had a trial spell with Leeds but has now agreed terms at The Valley.
"He's come to bolster the squad, he's a bit of everything," said Addicks boss Chris Powell.
"Jose Semedo has played every game for us and we don't have another player of that ilk. I felt we needed that sort of strong midfielder in there and he comes into that category."
Stewart, who has also played for Hearts, Hibs and Nottingham Forest, could make his Charlton debut at Bournemouth on Saturday.
Source: PA
Source: PA