Npower Canary Energy branding, alluding to Norwich City FC's 'Canaries' nickname, will appear on perimeter boards, match programmes and billboards around Norwich.
The company joins sponsors including Flybe - the main shirt sponsor - and Lotus. Norwich City FC is likely to receive a five-figure sum for each of the three years the deal will run.
E.ON renewed its sponsorship of Ipswich Town FC in April; its deal runs through to 2008.
An Npower spokeswoman said competing with E.On was not the only motivation for its sponsorship; it recently acquired a power station in neighbouring Great Yarmouth and wanted to integrate with the community.
Npower also has a four-year supply contract with Wembley's £450m stadium.