"T-shirts for Julian: you can show your support of a real hero by going to www.viviennewestwood.com," wrote Westwood, 71, who appears on the shirt wearing a denim jacket and a necklace with a skull pendant, with her red hair swept back.
Australian-born Assange, 41, walked into the embassy on June 19 and claimed asylum in a bid to avoid extradition to Sweden, where he faces questioning over allegations of rape and sexual assault.
Ecuador granted him asylum on August 16 but Britain has refused him safe passage out of the country, leaving Assange in the embassy as the two countries struggle to resolve a diplomatic stalemate.
Assange denies the allegations of sex crimes and claims he could eventually be passed to the United States if he is extradited to Sweden. WikiLeaks enraged Washington in 2010 by publishing a flood of classified US documents.
Flamboyant Westwood, a long-standing supporter of Assange, was pictured wearing the new t-shirt as she paid the WikiLeaks founder a visit last Tuesday.
US pop star Lady Gaga also went to see him earlier in October.