Amey has won a six-year contract worth £18m per year with Welsh Water.
The deal, which will see the firm maintain the network of clean water pipes in south Wales, starts in October 2013.
Amey, which is a subsidiary of Spanish contractor Ferrovial, will repair and maintain about 20,000km of pipes and handle new connections and meter installation in Pembrokeshire, Ceredigion, Herefordshire and Glamorgan.
Amey was recently selected by the Highways Agency to manage and maintain approximately 965 km of roads in East Anglia in a five-year deal worth £200m. The project is the agency’s Area 6 contract.