Galliford Try has won a £47m contract to redevelop Parc Prison in Bridgend, the only private prison in Wales.
The project will include developing a new accommodation block at the Category B prison, as well as extensions and conversions to existing buildings on the site.
The scheme has been designed to be BREEAM Excellent.
Work on site will start in May this year and completion is expected by February 2011.
Chief executive of Galliford Try, Greg Fitzgerald, said the contract built on Galliford Try’s experience in delivering custodial and public sector projects.