The agency won the natural search strategy account in a "competitive, multi-agency pitch", but the number of agencies involved has not been released.
Debenhams is hoping to hit its 2014 growth targets by working with iCrossing on search engine optimisation to improve its visibility and ranking in search page results.
iCrossing has implemented the early stages of this strategy and will soon begin working with Debenhams’ internal marketing and SEO teams. The group began working with the retailer in September.
Tom Jones, chief client officer at iCrossing, said: "We’re hoping it will be a long-term partnership, that’s our aspiration. It’s an exciting time at the moment in the run-up to Christmas, and making the most of the seasonal demand is very important.
"Into next year, we’ll be making sure we’re exposing as much of Debenhams’ inventory online as possible, to keep its business growing online."
Debenhams online revenue has grown by 46 per cent in the year to 31 August, from £251 million to £366 million. This means online trading now accounts for 13.2 per cent of all the retailer’s sales.
ICrossing’s other clients include The Coca-Cola Company, Barclays and LEGO.
In October, Debenhams appointed Forward3D to its paid search business.