US entertainment magazine Variety first reported on Monday that Braun had sold the rights - known as masters - to an investment fund.
It said the deal is thought to be worth more than $300 million (£227m).
Writing on Twitter Swift said it was "the second time my music had been sold without my knowledge".
It is the latest development in the dispute between Swift and Braun - Swift has previously accused Braun of trying to "dismantle" her musical legacy.