A Downing Street spokeswoman said: "The PM is officially moving in today and, yes, his partner will be living there."
Ms Symonds, a former Tory spin doctor, will not have any publicly-funded staff working for her, Number 10 indicated.
They will be the first unmarried couple to officially live in Number 10.
The couple had been living together at Ms Symonds's flat in Camberwell, south London, until a well-publicised row recorded by neighbours last month.
As Theresa May left Downing Street last week, removal vans were seen taking away her belongings.
Both Mrs May and former chancellor Philip Hammond left their flats in Downing Street as the handover to Mr Johnson's new administration took place.
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