Dunn, who has held the top finance job at Britvic since 2015, will remain in the role until the end of his notice period in April 2019.
Britvic shares have gained around 10 percent in value since he succeeded John Gibney as CFO in November 2015.
Dunn takes up the job at ASOS currently held by its Chief Executive Nick Beighton after Helen Ashton, CFO since September 2015, retired from the board on April 30.
ASOS is striving to take full advantage of an acceleration in internet shopping, with Beighton promising last month that the company’s had the potential to be “much bigger” than a 4 billion pound business in terms of annual revenue.
Britvic reported problems with supply disruptions in Britain over the summer and is still dealing with uncertainties for its business around the country’s new sugar tax.
Reporting by Shariq Khan in Bengaluru; Editing by David Goodman