Niall's 19.2 million followers immediately responded by retweeting and favouriting the image thousands of times. The terms "#HappyBirthdayTheo" and "#NiallTakeASelfieWithTheo" also started trending on Twitter.
Keen to satisfy his fans even more, the Irish heartthrob posted another photo of baby Theo sitting in a life-size black convertible toy car. "They see me rollin! They hatin!" Niall captioned the image.
The star's nephew has gained somewhat of a celebrity status, and even has a Twitter account dedicated to him that has amassed 255,000 followers.
Presumably run by the baby's parents Greg and Denise, the account keeps fans updated of the one-year-old's progress with photos and posts.
"Theo loves listening to one direction music. We think he will be a singer," was one recent post, followed by, "My mammy and daddy said I'm having a bouncy castle for my party on Saturday!!!"
Niall and his One Direction bandmates – Harry Styles, Louis Tomlinson, Zayn Malik and Liam Payne – are currently on their Where We Are tour. The boys have performed in South America, Europe and the UK.