It has also emerged that former global marketing director Clare Dobbie, who left Asos in December, has joined upmarket fashion retailer Hobbs.
Westlake will head up the online retailer’s brand and digital marketing and advertising, including content creation, communications, PR and CRM.
She will hold the role on an interim basis as Asos looks for a replacement to fill the role permanently.
A spokesman for Asos says: “As we continue the process of recruitment to fill the position of marketing director, current strategist Terri Westlake will take the role of interim marketing director, reporting directly to Nick Robertson.
“With a wealth of strategic and operational experience, Terri will be leading the marketing team to develop the ASOS strategy for 2012/13.”
Asos’s UK sales have slowed in the past year as the group looks overseas to drive growth. Total sales in the final three months of last year were 46% up on the previous year to £146m, driven by international sales which now account for 58% of the business.
Dobbie, who was at Asos for less than two years, has taken on the marketing director role at Hobbs, where she will look after the clothing retailer’s brand, marketing and online teams across its Hobbs London, NW3, Invitation and Unlimited brands.
She will also spearhead Hobbs’ international expansion plans and its multichannel strategy, which includes an £18m investment to improve customer experience in-store and online.
Dobbie was previously a consultant for ad agency Mother and vice-president of marketing for Europe at Gap where she helped launch US brand Banana Republic in London.
At Asos, Dobbie was responsible for driving the expansion of Asos overseas and oversaw the launch of its ecommerce operations in Australian and American sites and its German, French, Italian and Spanish language sites as well as digital initiatives such as a mobile site, iPhone and iPad apps, Facebook store and Asos Marketplace.