The 20-year-old's contract was due to expire this summer but he has now signed a deal to stay at St Mary's until the summer of 2012.
Doble - who can operate in midfield or attack - has yet to make his first team debut for the Saints but has impressed for the academy side and reserves.
He has also previously been called up by the Wales senior team, having been included in the squad for the Euro 2012 qualifying matches against Bulgaria and Switzerland in October.
Source: PA
Source: PA