She will join to lead the 350 strong product and trading team in January 2013.
Bostock left M&S in July after eight years with the high street retailer. Previously she held senior roles at George at Asda and Next.
Nick Robertson, Asos CEO, says: “We are fortunate to have Kate bring us her wealth of experience from some of the biggest names in retailing as we strengthen our retail disciplines and put in place the structures needed to manage the next phase of our growth.”
Bostock says: “Asos is a phenomenon and I’m in awe of the story. With its unique mix of own label and fashion forward brands delivered online and increasingly through mobile, the potential for Asos globally is huge. I’m very excited to be joining at such a time of immense opportunity”.
Robert Bready, current product director is leaving Asos after seven years to “pursue fresh challenges”.
The online retailer also appointed former Amazon UK managing director Brian McBride as chairman and executive director last week.