The client is Bridgend Custodial Service Ltd, in co-operation with G4S Care & Justice Services (UK) and the Ministry of Justice.
The £35m contract will involve the creation of a new house block containing 216 cells for 387 prisoners.
Galliford Try will also build a two-storey industrial workshop and education building, along with a visitor centre and gatehouse. Works also include a new 350-space car park, extended perimeter walls and security fencing.
The same contractor completed phase one in 2011 under a £47m contract
Galliford Try chief executive Greg Fitzgerald said: "We are delighted to have been selected again by G4S and the MoJ following the successful completion of phase one at HMP Parc. This contract demonstrates the difference our proactive approach to partnering with our clients and delivering their requirements makes in a competitive market place."