The "Harry Potter" author said in a statement on her website Wednesday she had decided that "while independence might give us opportunities ... it also carries serious risks."
Scots will vote in a Sept. 18 referendum on whether to leave their 300-year-old union with England and become an independent country.
Rowling said she was particularly concerned about the impact on the economy and on medical research.
The author, who has lived in Scotland for two decades, said she had made a "substantial donation" to the anti-independence "Better Together" campaign. Spokesman Mark Hutchinson confirmed the donation was 1 million pounds.
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