The advertising campaign is the Redmond-based company’s attempt to fight off increasing competition from Apple.
Steve Jobs’ Apple firm has managed to all-but-kidnap the ‘cool’ status in the industry thanks to products such as the iPod and iPhone.
Seinfeld will be paid $10m in exchange for appearing in ads with Microsoft’s chairman Bill Gates.
Microsoft has refused to comment on the deal, with a spokesperson only saying, "Stay tuned, literally".
The advert is expected to debut on 4 September in the US.
The adverts obviously won’t be true to Seinfeld’s character in the hit show, as he had an Apple Mac in the background of his apartment…