The Harry Potter author, who was born in England but has lived in Scotland for 21 years, gave the money to Better Together, explaining her decision in a statement posted on her website.
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JK Rowling has donated £1million to the Better Together campaign
She also correctly predicted that she would be targeted by a "fringe of nationalists" who "like to demonize anyone who is not blindly and unquestionably pre-independence", comparing them to the Death Eaters from her books.
Writing that many would deem her "insufficiently Scottish to have a valid view", JK wrote, "When people try to make this debate about the purity of your lineage, things start to get a little Death Eaterish for my taste".
The author - worth an estimated £1billion - explained, "My hesitance at embracing independence has nothing to do with lack of belief in Scotland's remarkable people or its achievements.
JK Rowling is now the largest financial backer of the campaign to keep Scotland in the UK
"The simple truth is that Scotland is subject to the same 21st century pressures as the rest of the world. It must compete in the same global markets, defend itself from the same threats and navigate what still feels like a fragile economic recovery.
"The more I listen to the 'Yes' campaign the more I worry about its minimisation and even denial of risks. Whenever the big issues are raised - our heavy reliance on oil revenue if we become independent, what currency we'll use, whether we'll get back into the EU - reasonable questions are drowned out by accusations of 'scaremongering'."
Voters in Scotland will go to the polls on 18 September to decide on their country's future and will be asked the Yes/No question, 'Should Scotland be an independent country?'