The agency has been hired to handle the website's paid-for and natural search campaigns. It follows the appointment last year of Steak Media to run paid search campaigns for John Lewis.
Steak Media will be charged with campaign analysis to integrate those elements of online activity with other Greenbee.com marketing channels.
Rick Jones, head of marketing at Greenbee.com, said the agency had won the business because of its impressive previous work with the company.
Steak Media, which specialises in search engine marketing, as well as media planning and buying, has a client list including British Gas, Time Out and 118 118.
As well as opening offices in Melbourne and New York, earlier this month the company announced it was becoming a full-service agency, with the launch of Steak Creative, hiring Tesco.com's head of design John Beach to lead the division.
Greenbee, part of the John Lewis Partnership, offers services from a range of partners such as AXA and Expedia.