Topshop had sold the t-shirt - which bore an image of Rihanna taken from a photograph - between March and August 2012, Mr Justice Birss was told.
The High Street store had no "licence" from Rihanna to sell the shirt. And the singer had taken legal action claiming that selling it without her permission infringed her rights.
Topshop disputed her claim saying nothing on labelling suggested that the t-shirt had been authorised by Rihanna.
And bosses said the public had no expectation that clothes were authorised by people shown in an image.
But Mr Justice Birss today ruled in Rihanna's favour - after analysing evidence at a trial in London earlier this month.
The judge said Topshop's sale of the t-shirt without her permission was "passing off".
"I accept that a good number of purchasers will buy the t-shirt without giving the question of authorisation any thought at all, in my judgment a substantial portion of those considering the product will be induced to think it is a garment authorised by the artist. The persons who do this will be the Rihanna fans," said Mr Justice Birss.
He said a "substantial number" of buyers were likely to have been deceived into buying the Rihanna T-shirt because of a "false belief" that it had been authorised by the singer.
The judge said that was damaging to her "goodwill" and represented a loss of control over her reputation in the "fashion sphere".
He said it was for the singer not Topshop to choose what garments the public thought were endorsed by her.
And he added: "I find that Topshop's sale of this Rihanna t-shirt without her approval was an act of passing off."
The judge made no assessment of damages in today's written ruling - he only weighed arguments about whether the use of the image had been justified. Topshop said later that damages had yet to be assessed.
A Topshop spokeswoman said bosses were "surprised", "disappointed" and "perplexed" by the ruling - and considering an appeal.
"There was no intention by Topshop to create the appearance of an endorsement or promotion," said the spokeswoman. "We do not believe it conveyed any false message to our customers."
She added: "There was no evidence of consumer confusion to support the judge's conclusion.
However it does appear that in this case Topshop's reputation and popularity has worked against us."