The 17-year-old defender, who was previously courted by a number of Football League teams, recently moved to the Saints youth academy from Plymouth Argyle for ?150,00.
However, Stephens cannot play for the first team this season having signed outside the transfer window.
Five other academy graduates have also been handed professional deals for next season.
Youth team captain Andy Robinson, wide midfielders Lloyd Isgrove and Omar Rowe and strikers Alberto Seidi and Sam Hoskins have all signed contracts and will join the first-team squad for the 2011/2012 season.
Source: PA
Source: PA