The prime minister responded to eight-year-old Monti, telling them he'd "put a call in to the North Pole" and Father Christmas is "raring to go, as are Rudolph and all the other reindeer".
He said: "I have spoken with experts and can assure you that Father Christmas will be packing his sleigh and delivering presents this Christmas!"
Mr Johnson passed on a message from Chief Medical Officer Chris Whitty that, "providing father Christmas behaves in his usual responsible way and works quickly and safely, then there are no risks to your health or his".
In the letter, Monti said: "I am eight years old and was wondering whether you and the government had thought about Santa coming this Christmas.
"If we leave hand sanitiser by the cookies, can he come?"
She added: "I understand you are very busy but can you and the scientists please talk about this?"
The PM said her hand sanitiser idea was an "excellent" one and that frequently washing her hands herself is "exactly the kind of thing that will get you and your friends on the nice list".
Mr Johnson's letter follows news that coronavirus restrictions will be relaxed at Christmas, allowing three separate households to mix for five days.