Morgan Sindall fought off fierce competition from Balfour Beatty to win the Infrastructure Strategic Alliance contract.
Work will include the the maintenance of steam and electricity generation, water supply, chemical storage and distribution, drainage networks and all transport infrastructure at the site.
The contract is expected to be signed later in the year, for an initial five year period, with an option to extend the contract for a further 10 years.
Paul Smith, Chief Executive of Morgan Sindall Group, said: “This appointment is fantastic news for the Group and Sellafield as a whole.
“The award builds on our strong track record of delivering on the most complex and challenging infrastructure projects for our clients.
“The renewal of Sellafield’s infrastructure is an important commitment to Cumbria, and underlines the Group’s continuing focus on UK regeneration.”
Nuclear Management Partners’ Deputy Managing Director for Sellafield Ltd, George Beveridge, said: “We have set ourselves a number of challenging targets through our performance plan.
“Long term, collaborative alliances with supply chain partners represent best industry practice and it’s important that we seize that opportunity to unlock expertise and innovation and safely deliver our work programme quicker and more cost effectively.
“We’re confident that in Morgan Sindall and Arup, we have found partners with a proven track record that will help us ensure that our infrastructure assets receive the best possible care.”