Usher is "proud" of the man Justin Bieber has become.

Justin Bieber

Justin Bieber

The 'Sorry' hitmaker celebrated his 23rd birthday on Wednesday (01.03.17) and his long-time mentor has taken to Twitter to pay tribute to Justin, admitting he's thrilled by how the chart-topping star has matured in recent years.

Usher wrote: "Happy Birthday @JustinBieber !! I'm proud of the man you've become. #JB23 (sic)"

The 38-year-old singer-songwriter's sentiments were echoed by Justin's manager Scooter Braun, who has played a central role in his success.

Scooter, who also manages Ariana Grande, wrote on Twitter: "To another 23 years...keep shining your light onto the world . #jb23 #mybrother happy birthday @justinbieber (sic)"

Other big-name stars who sent their best wishes to Justin included DJ Khaled, Akon and LL Cool J.

Khaled wrote on Instagram: "Happy bday the YOUNG ICON! @justinbieber ! Bless up young [king]! THANK YOU SO MUCH FOR YOUR FRIENDSHIP ! #GRATEFUL (sic)"

Akon said on Twitter: "Happy birthday @justinbieber! Keep doing your thing & enjoying life #JB23 (sic)"

And LL Cool J wrote on the micro-blogging site: "Happy 23rd to the man @justinbieber!!! (sic)"

Meanwhile, another of Justin's showbiz pals, Ed Sheeran, recently revealed that he duetted with the Canadian star at a karaoke bar in Tokyo, Japan.

Ed admitted the celebrity duo had a "weird" night out where they sang one another's biggest hits before coming together for a rendition of 'Love Yourself', Justin's 2015 single which was written by the 'Castle On The Hill' hitmaker.

Ed recently shared: "I quite like that that song is just having its own life somewhere.

"Me and Justin actually sung that on karaoke together two days ago. He was in Tokyo. It was a really weird night actually, a really weird night.

"We went to a bar called Train Bar and did some shots, then we went to a karaoke bar and sung our own song. He was singing my songs, so I was singing his songs ... and then we both sang 'Love Yourself'."