The former Aldershot and Barnet defender left the Cobblers on Thursday morning after almost at year with the club and was announced as Argyle’s second signing of the day just hours later.
The 31-year-old, who has also played for the likes of Crewe and Hyde, has made 11 appearances this season.
Charles is the club’s second acquisition of the day after Reading striker Gozie Ugwu signed a loan deal at Home Park on Thursday.
Gary Rowett's injury crisis shows no sign of easing as Derby prepare to play host to Championship promotion hopefuls Cardiff.
It's 15th vs 18th at the John Smith's Stadium on Saturday, as Huddersfield and Crystal Palace meet in a crucial battle at the bottom.