The 32-year-old, who has been assigned squad number 99, passed a medical on Wednesday afternoon and has agreed a deal with the Buddies until the end of the season.
The 24-time capped Cameroon international joined United in 2003 but failed to live up to his potential and later moved to Aston Villa.
He has since notched up spells at Qatar SC, Danish side Odense, Hapoel Tel Aviv in Israel before moving to Partizan Belgrade last summer.
However, his stint in Serbia lasted just six months as he was released after making 14 appearances. He could make his saints debut in Sunday's William Hill Scottish Cup clash with Dundee United.