The five-figure agreement is part of an integrated campaign to promote two new games - Sonic the Hedgehog on X360 and Sonic Rivals on PSP.
The campaign, which targets 13- to 34-year olds, aims to raise the profile of the brand by allowing users to interact with Sonic and download backgrounds using the Windows Live theme pack.
Tina Hicks, head of marketing at Sega UK, said: "Digital advertising is a key medium for Sega, particularly as our target audiences reside online and are looking for information to support their buying decisions."
Sonic has sold in excess of 44m games since its first appearance on the Sega Mega Drive in 1991.
"We have a wide audience to reach and converse with. The MSN theme pack enables users to interact with the Sonic brand in a way that is not possible with standard online advertising or rich media, which can be a far more passive way of talking to consumers," Hicks said.
The deal also includes banner advertising and a promotion to drive traffic to the Sega website, where users can enter a competition to win a range of prizes.
Katy Adelson, digital marketing manager of Microsoft Digital Advertising Solutions, said: "This is another great example of how the MSN theme packs provide brands with a highly targeted platform to achieve creative standout and cut through, whilst leaving a lasting impression on the user.
"This kind of interaction increases the user experience and puts the consumer in control as they are able to interact with the brand as and when they want. This is one of the key challenges for advertisers; to present their brand to the right target audience in a creative way to encourage consumers to interact with, rather than ignore."
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