Chris Gossage of London law firm Russells Solicitors - which represents Rowling - told a friend of his wife that the "Harry Potter" creator was author of "The Cuckoo's Calling," published last year under the name Robert Galbraith. The friend tweeted the information, and it was followed up by the Sunday Times.
The law firm apologized and paid damages to Rowling for the leak.
The Solicitors Regulation Authority says it issued Gossage with a written rebuke for disclosing confidential information about a client to a third party.
The ruling was dated Nov. 26 but published this week.
Sales of the thriller rocketed after Rowling was outed as its author.