Royal Mail has tasked Computacenter with providing desktop services for 30,000 of its staff.
The outsourcer will manage IT for the service desk, security operations centre and distributed desktop services to improve user experience for Royal Mail employees.
Royal Mail, partly privatised last year, hopes the deal will improve efficiency and service performance across its head office, sorting and distribution locations.
Catherine Doran, Royal Mail CIO, said: "Delivery companies are technology-led companies and IT transformation throughout Royal Mail is central to the company's parcels and letters business.
"The transition to our new IT suppliers will help promote innovation and ensure best practice is embedded into our IT function.
"This will provide the technological backbone required to deliver further innovations for our customers. Computacenter will perform a vital role in this delivery."
The contract is part of Royal Mail's wider IT transformation programme, and it hopes to inject competition into its IT services provision by increasing its number of suppliers to get greater value for money.
It already announced a 130 million investment in October 2014 to provide postmen and women with 76,000 handheld devices to improve the delivery of letters and parcels.
BT will cover the day-to-day management of those devices under a five-year deal.
Computacenter did not reveal the length or value of the contract, but signed an IT transformation deal with the Post Office in December 2014 that TechMarketView valued at upwards of 100 million.
That four-year deal will see Computacenter roll out more than 30,000 tablets and PCs to staff.
John Beard, director of Computacenter's financial services and retail sector, said: "We understand that IT will play a critical role in the transformation of Royal Mail. We will be implementing and managing new workplace technologies to help them maintain service excellence and further enable their users."
