The book had sold 1,500 copies before the secret emerged in the Sunday Times. Within hours, it rose more than 5,000 places to top Amazon's sales list.
Rowling said she had "hoped to keep this secret a little longer".
The author described "being Robert Galbraith" as a "such a liberating experience".
'Sequels plan' "It has been wonderful to publish without hype or expectation, and pure pleasure to get feedback under a different name," she said in a statement.
Rowling said her editor, David Shelley, had been "a true partner in crime".
High Street booksellers appear to have been caught completely unaware by the announcement, and are finding their meagre stocks are unable to match demand.
"We had one copy, but we sold it last week," says lead bookseller Holly Popple of Waterstones in Piccadilly, central London. The few other copies scattered around sister branches have now also been sold or reserved, she reports.
Hatchard's too, a nearby bookshop dating back to 1797, is bereft of copies: the few they had trickled off the shelves in recent weeks.
Meanwhile Brett, from Daunt Books in Marylebone High Street, says: "We had one or two copies this morning, but they've all disappeared by now.
"Everybody's after it."
A clue that Rowling was behind the novel was that she and "Galbraith" shared an agent and editor.
The book was published by Sphere, part of Little, Brown Book Group which published her foray into writing novels for adults, The Casual Vacancy.
Crime writer Peter James told the Sunday Times: "I thought it was by a very mature writer, and not a first-timer."
While crime author Mark Billingham, who reviewed the book ahead of its publication in April, said he was "gobsmacked" at the revelation.
Rowling also caught out others with her new guise.
Turned down Kate Mills, fiction editor at Orion Books, admitted she had turned down the crime novel, which she described as "well-written but quiet".
Waterstones spokesmanThis is the best act of literary deception since Stephen King was outed as Richard Bachman back in the 1980s”
The "Movers and Shakers" section of Amazon, which charts gains in sales by the hour, says sales of the book are currently up by more than 507,000%.
A spokesman for Waterstones booksellers said: "This is the best act of literary deception since Stephen King was outed as Richard Bachman back in the 1980s."
In a tweet, its Oxford Street branch joked: "SPECIAL OFFER: For today only, ALL of our books were written by JK Rowling!"
Others also took to Twitter to react to the news.
Comedian Michael Moran posted: "Idea for publishers: 1: Reveal that ALL books were written by JK Rowling. 2: Sales of all books soar by 150,000%. 3: Industry saved."
Author Ian Rankin wrote: "So a debut novelist, garnering good quotes from famed authors for the cover plus good reviews, can expect to sell only a few hundred copies."
While an account named Dumbledore's Beard TM posted: "JK Rowling is a genius and proper badass."
And another named the Dark Lord tweeted: "JK Rowling secretly wrote a book under a different name. How very Half-Blood Prince of her."
The fictitious Galbraith was supposed to have been a former plain-clothes Royal Military Police investigator who had left the armed forces in 2003 to work in the civilian security industry.
In previous interviews, Rowling has said she would prefer to write novels after Harry Potter under a pseudonym.
Another Cormoran Strike book by Robert Galbraith is in the pipeline, to be published next year.