The agency, which already handles advertising for Fiat's mainstream marques, will take on all the advertising for Fiat's range of commercial vehicles, with its first work expected to break in March.
Fiat manufactures a number of vans and passenger carrying vehicles up to 4,000kg in weight, and is now seeking to bolster the range.
Gerry Clarke, the director of Fiat Professional UK, said: "The pitch asked for a radical approach to marketing in a sector that has been hit hard by the recession, and is incredibly competitive.
"Krow delivered an idea that challenged us on pretty much every level, and we are very excited about getting it out into the marketplace."
The appointment comes one month after Fiat moved its estimated £20m UK media account out of Mediaedge:cia and into Maxus.