Imogen Challen, 20, asked pals to dare her to do things to celebrate her birthday in May.
The University of Edinburgh student, who is studying Philosophy in her second year, wanted to raise money for NHS Charities Together.
Her friend Brock Rodgers, 20, asked her to get BoJo's name tattooed on her leg - which Imogen refused but agreed to get it on her bottom.
But she got inked with the PM's name on her right bum cheek, on Tuesday.
It was her first tattoo and cost £40.
After agreeing to get that tat - which friends refused to believe she would go through with - donations soared from £400 to £1,000, with donations pouring in from her hometown, Doncaster.
Imogen said: “My friend Brock replied to one of the polls saying ‘get Boris Johnson tattooed on your leg’, but instead I agreed to get it done on my a**e.
“I was at £400 on May 4 and taking suggestions, but the next day we’d hit a grand.
“Everyone got involved and it just kept going.
“It was a pretty even split between people from home and Edinburgh, and people were getting their friends to donate as well - it was great.
“I think everybody thought I wouldn’t do it but I just couldn’t because of Covid.
“I would’ve never not done it.
“I’m glad I did it - the money’s going to a really good cause.”
She raised a total of £1,594.