Scottish Water has extended Fujitsu's contract to provide IT support services by two years to August 2018.
The extension means Fujitsu will continue to support 4,500 employees of Scottish Water and its partners, managing day-to-day issues and support for 4,600 devices.
Those include 2,700 laptops and 1,200 desktops, as well as 700 tougher devices designed for the water services body's field workforce.
The contract extension comes after Fujitsu won the initial eight-year deal back in 2008.
The infrastructure management part of the contract tasks the tech giant with looking after 1,400 servers, 200 databases and 20 storage area networks.
Mark Dickson, director of technology and information at Scottish Water, said: "We are pleased to be continuing our partnership with Fujitsu and look forward to ensuring that technology enables Scottish Water to build on our improved performance for the benefit of customers."
Fujitsu is one of three firms working with Scottish Water as part of a Master Services Agreement covering much of the organisation's IT.
BT is providing network and communication services while Tata Consultancy Services is responsible for application development.
Scottish Water claims the agreement will help cut operating costs by 1 million a year.
Jeph Hamilton, of Fujitsu UK & Ireland, said: "Service quality, competitiveness and commercial sensibility are central to our role as a key IT partner of Scottish Water, and we're honoured to help them define and deliver on their ongoing IT strategy."
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