In a brief statement, the AIM-listed group said Ashton, its finance chief since September 2015, will retire from the board on April 30 but remain an employee of the company until August 31 to ensure an orderly handover.
The firm said its chief executive Nick Beighton, who was previously CFO of the retailer, will cover Ashton's role until her replacement is identified.
ASOS did not say if Ashton has another job to go to.
Back in January, the online retailer revealed that its sales soared over the Christmas period, driven by an “exceptional performance” in its core UK market.
ASOS' UK sales grew by 18%, with shoppers taking a particular liking to a new same-day delivery option and a ‘Try Before You Buy’ feature this time around.
The group’s total sales – which included contributions from the US, Europe and the rest of the world – jumped 30% to £790.4mln (2016: £605.7mln), above the top end of its guidance
In line with its plans, ASOS said, gross margins were up 80 basis points year-on-year.