Computacenter will be responsible for providing service desk, security operations and distributed desktop services across Royal Mail's headquarters, sorting and distribution offices.
Royal Mail said the deal will increase efficiency and service performance and improve the user experience for its staff.
It is part of a wider IT transformation programme being introduced across the newly-privatised postal service firm, which hopes to save money and improve services by using a more competitive multi-supplier service 'Towers' approach.
Royal Mail is also investing £130 million to hand out 76,000 handheld devices to postmen and women between now and 2017 as part of a contract with BT.
The firm promised it would help to support more flexible deliveries for customers not at home when it attempts to deliver parcels, and will make it easier for customers to track their goods.
Last year Royal Mail launched a new parcels shipping and tracking application programming interface (API) platform to allow e-retailers to integrate their IT systems with the postal firm.
This would allow customer delivery information to be shared between retailers and Royal Mail, so customers' designated 'safe places' for when they're not in can be sent to posties' handhelds.
Royal Mail CIO Catherine Doran 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," Doran added.
Computacenter did not disclose the length or the value of the contract when asked by ComputerworldUK.
Sign up for Computerworld eNewsletters.