Creature picked up the business after a pitch against Now, 101 and Dentsu.
The agency will be responsible for producing the company's first above-the-line ad campaign.
The appointment of Creature follows Moo.com's recent tie-up with Facebook to create Facebook Cards, which enables users to create personalised business cards based on images and posts from their profile.
Paul Lewis, Moo.com's head of marketing, said: "We were looking for a young, entrepreneurial agency that reflected both the dynamism of our fast-growing company and the exceptional entrepreneurial verve of Moo.com's customer base. That and a great idea. We found all of that in Creature."
Dan Shute, the managing partner at Creature, said: "Moo.com is a genuinely remarkable company."