The Irish singer had an operation in the US to correct a long-standing problem with his left knee ahead of the band's next tour in May.
On Thursday, he was pictured for the first time since undergoing the procedure as he arrived at London's Heathrow Airport.
Niall was seen hobbling on crutches as he left the arrivals terminal and headed to a waiting car.
He later took to Twitter to inform fans he had undergone "major" surgery, but assured them he is on the mend.
After a series of posts, he added: "So all I ask of you guys is that you would respect my privacy as I try to recover and get ready for the Where We Are tour!"