The 27-year-old, who joined the Pilgrims in September from Port Vale, has linked up with one of his former clubs on a permanent basis.
Archer made four substitute appearances for United and moves on with the club's best wishes for the future.
Reflecting on Archer's sale, United boss Craig Elliott said: "I am disappointed to lose Jordan as I genuinely think he has all the attributes to do well.
"However, he made it very clear that his priority this season was to be playing games and at this stage, he could not see himself breaking into the team on a regular basis.
"When Southport made us an offer, it seemed the right thing to do for both the player and the club and we wish him all the best."
PIC: Russell Dossett.