The deal follows an earlier contract with Rackspace from 2004 to host the army's recruitment website and ArmyNet, the military's online social networking service.
The new deal consolidates web hosting for the other two services, saving the Ministry of Defence (MoD) around 26% in total costs in the process, said Fabio Torlini, marketing director of Rackspace.
Fabio Torlini said the deal did not include the military's operational or in-theatre communications, although frontline troops have access to ArmyNet.
ArmyNet carries news about sports, associations, museums, health and safety, as well as a Twitter feed for breaking non-operational news to tens of thousands of servicemen and women.
Torlini said Rackspace had set up a dedicated secure virtualised server and storage system for the services.