Under the agreement, Thames will supply both water and sewerage services to customers in the garrison town of Tidworth in Wiltshire.
Thames has signed a 20-year contract with the Ministry of Defence to supply both the military and civilian population of Tidworth - in an area served by Wessex Water and Southern Water.
This would be first "inset" appointment for both water and sewage, whereby a company takes over supplies in another franchise area.
Anglian Water has a contract to supply water only to a chicken farm outside its own area.
Ian Byatt, director general of water services, said the Thames agreement would increase competition in the industry.
The MoD owns and operates the water system in Tidworth for the barracks as well as the civilian population. The site is not connected to either Thames' main water or sewerage system.