Niall Horan has joked he's "delighted" after sprouting two more chest hairs following his 20th birthday celebrations.

The One Direction star left behind his teenage years on September 13.

Horan took to Twitter today to thank well wishers for their birthday messages, and joked about his ever-growing masculinity.

He quipped: "Thank you all for your birthday wishes yesterday! Means alot! woke up this morning with 2 more hairs on my chest! Delighted with meself (sic)."

Horan received floods of messages on his birthday, with stars including Olly Murs and Rochelle Humes tweeting the chart-topping singer a great day.

Murs wrote: "Happy birthday @NiallOfficial have gooden dude wherever you are in the world... Catch up soon!! (sic)."

New mom Humes tweeted: "@NiallOfficial Happy Birthday Son ❤ Big kiss xx (sic)."

Horan's bandmate Harry Styles wished his bandmate a great day before making a cheeky reference to putting on a gamble.

He joked: "Happy Birthday @NiallOfficial ... Behave yourself. All on red. (sic)."

Horan and the rest of the group are currently enjoying a well-deserved break from One Direction's gruelling Take Me Home world tour. The Australia leg kicks off on September 23 in Adelaide.

The heartthrobs are also set to launch their third studio album 'Midnight Memories' in November.