Shares fell more than 1% after the FTSE 100 company announced that product director Christos Angelides was leaving to join US retailer Abercrombie & Fitch.
Mr Angelides has worked for Next for 28 years and is seen as a key part of the success which this year saw it record annual profits of £695 million, beating high street rival Marks & Spencer for the first time.
He is the second director to signal his departure in less than a month, with long-serving finance director David Keens announcing in May that he is to leave next year.
Mr Angelides, 51, will continue working at Next until he leaves in September, the retailer said.
"Christos has made a huge contribution to the group in his 28 years of service and the board wish him success in his future endeavours," it added.
Mr Angelides is expected to take up his position as president of the Abercrombie & Fitch and Abercrombie kids brands in October, the US firm said.
Educated at De Montfort University in Leicester, he has spent his entire career at Next, with roles including general manager at its sourcing office in Hong Kong, womenswear product director, and menswear product director, before becoming group product director in August 2000.
Abercrombie chief executive Mike Jeffries said: "We are excited to welcome Christos to the Abercrombie & Fitch team and to deepen our bench of senior leadership talent.
"His experience with all aspects of running a business made him the perfect candidate for this newly-created role."
Mr Angelides said: "I am honoured to join one of the most iconic apparel brands in the world."