Writing on a website set up for Galbraith, the British author confirmed that she has just finished a sequel, which will be published under his name next year.
Rowling, who has sold more than 450 million copies of the Harry Potter books, was accused of planning her own unmasking as a publicity stunt -- but she insists she had wanted her identity to stay secret.
"If anyone had seen the labyrinthine plans I laid to conceal my identity (or indeed my expression when I realised that the game was up!) they would realise how little I wanted to be discovered," the 47-year-old wrote.
The author described how she spent days working on Galbraith's signature for the relatively few signed copies that are now in circulation.
"My Robert Galbraith signature is distinctive and consistent; I spent a whole weekend practising it to make sure," she said.
British legal firm Russells apologised to the mother-of-three last week after it discovered that one of its partners was the source of the leak, having told his wife's best friend "in confidence" about Galbraith's true identity.
Sales of the book -- the tale of a private detective investigating a model's apparent suicide -- have risen an astonishing 41,000 percent in the last week, according to data released by Nielsen BookScan.
But Rowling said Galbraith hadn't needed her help -- his early sales compared "favourably with J.K. Rowling's success over the equivalent period of her career".
Two television companies had also expressed an interest in adapting the novel before its true author was revealed, she added.
Galbraith was originally described by the publisher as a former member of the Royal Military Police who has been working in the civilian security industry for the last decade.
This backstory provided him with "a solid excuse not to appear in public or provide a photograph", Rowling said.
She revealed she had chosen the name Robert after her hero Robert F. Kennedy, the US politician who was assassinated in 1968.
The reason behind the surname was more random: "When I was a child, I really wanted to be called 'Ella Galbraith', and I've no idea why," she wrote.
Rowling said she wrote under a pseudonym because, as one of the world's most famous authors, she had been yearning to "work without hype or expectation and to receive totally unvarnished feedback".
Her first novel for adults, "The Casual Vacancy", was published last year to mixed reviews.
Expectations had been high after the huge success of the seven Harry Potter books, which were made into blockbuster Hollywood movies and made Rowling a multi-millionaire.
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