Horan headed home to Mullingar in Ireland so he could be at his older brother Greg's side as he exchanged vows with his longtime girlfriend Denise Kelly.
The dapper brothers arrived at St Michael's Church in County Westmeath in the singer's Range Rover and greeted guests, as well as hordes of fans who had gathered at the gates of the church to catch a glimpse of the teen heartthrob.
Before the big day, Greg, 25, revealed his superstar sibling had always been his first choice to take on the role of best man on his big day.
He told the Irish Sun, "He has been working on the speech as far as I know, so we will see how that goes.
"He’s my only brother - there was never any other choice for my best man than Niall."
Horan's One Direction bandmates were invited to the wedding, but it is not yet known if any of the stars attended.