Nick Robertson, chief executive at Asos, has announced Bostock is leaving the company after they both agreed "Asos is not the right platform for her talent".
The board is yet to decide if it will appoint a direct replacement for Bostock, who was responsible for overseeing a product and trading team of 350 people.
Robertson said: "Of course we are disappointed that things haven't worked out and Kate leaves with our very best wishes. In the meantime, we have a very strong team in place and the business continues to fire on all cylinders."
Bostock added Asos wasn’t "the right place for me" and has insisted she "will not regret the experience", while she praised the business for being "right at the cutting edge of young online fashion".
Bostock joined Asos in January 2013 following months of speculation she would quit her role as executive director of general merchandise at Marks & Spencer in order to join the online retailer.
She finally parted company with Marks & Spencer after eight years on 1 October "by mutual consent" in order to "pursue other interests".
The announcement was made on the same day M&S posted a 0.7% first-quarter sales decline on the back of diminished clothing sales.
M&S is hoping a long decline in its clothing offering will be reversed by its autumn/winter range, which has been well received by the fashion press and is being masterminded by former Debenhams chief executive Belinda Earl.