High Court Judge Birss ruled in Rihanna's favor following a four-day hearing earlier this month.
In March last year Topshop, which is owned by billionaire Philip Green's Arcadia Group, started selling a T-shirt with an image of Rihanna on it.
The image was a photograph taken by an independent photographer in Northern Ireland while Rihanna was filming a music video. Topshop had a license from the photographer but no license from Rihanna.
In his judgment Birss said "a substantial number" of purchasers were likely to be deceived into buying the T-shirt because of a false belief that it has been authorized by Rihanna.
"I find that Topshop's sale of this Rihanna T-shirt without her approval was an act of passing off. I find for the claimants," he said.
Topshop was not immediately available for comment.
By: James Davey; Editing by Paul Sandle