The Irish heartthrob secretly underwent a procedure to fix a knee problem in January, later telling 1D fans that everything had gone well.
Yesterday, the singer uploaded a photo of the scar left behind on his leg... and the after effects of naughty bandmate Louis Tomlinson attacking it with a marker pen.
"Haha @louis_tomlinson decided he was gona colour in my scar ! Looks mad," said Niall.
Announcing the surgery in January, the 20-year-old said: "Yes, I've had major knee surgery, a much larger surgery than first anticipated, but I would like to tell you it was a complete success! Considering the scale of it!"
He added: "As It was a very important time for me and I didn't want to have fans knowing my whereabouts Needed privacy obviously."
One Direction's brief hiatus at the start of the year allowed time for Niall to recover from the surgery (and take a breather following an insanely busy year), which is just as well as 1D kicked off their huge 'Where We Are Tour' last week.
The band – including Harry Styles, Liam Payne and Zayn Malik – launched their latest show to a sold-out crowd of thousands in Bogota, Colombia. Their next date is in Santiago, Chile on 30 April. They'll bring the explosive tour to the UK and Ireland next month.
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