The Scot was released by Brentford at the end of last term, after failing to accept a reduced terms contract offer, and was left scrambling for a new club.
The Shrimpers made him the most concrete proposal, and terms were agreed over a one-year deal at the weekend.
Fullarton passed his medical with no problems, allowing him to link up with Steve Wignall`s squad.
The 28-year-old midfielder has a wealth of experience, having played in England, Scotland and France, and he could prove to be an astute signing by Wignall.
