The 31-year-old, who was a fans' favourite at the Goldsands Stadium, has played against the Grecians for the Cherries as well as Aldershot, Stevenage and most recently Plymouth in the 1-1 draw in December.
With the news that Liam Sercombe will be out of action for a number of months, Exeter boss Paul Tisdale moved quickly to add the Londoner to his squad.
"Mark has always been a tough competitor whenever he's played against us and will offer us an immediate solution to the void left by the absence of Liam on the right of midfield," Tisdale said.
"He will bring strength and know-how to the team."
Molesley began his career in the non-league game before becoming one of Eddie Howe's first signings for Bournemouth in January 2009 after a successful loan spell under the previous management.
He was a crucial part of Howe's team that pulled off the great escape that season and he would go on to make 60 appearances for the club, scoring five times.
After recovering from injury problems Molesley made 14 appearances for the Cherries last season and spent a spell on loan back at Aldershot before a six-game loan stay with City's Devon rivals Plymouth earlier in this campaign.