Leah Hession, six, was given the chance to meet her idol prior to attending the band's gig at Croke Park in Dublin in May.
The youngster lost her battle with a rare form of kidney cancer this week, and Horan has mourned her loss in a series of posts on Twitter.
The Irish singer shared a picture of himself hugging Leah along with the message, "RIP Leah , it was an honour to have met you! Very brave young lady! Thoughts are with Leah's family at this time!"