5 Seconds of Summer rocker Luke Hemmings is engaged.

Luke Hemmings

Luke Hemmings

The 'Youngblood' hitmaker took to Instagram to share a series of professional snaps from the romantic moment he got down on one knee and asked his longtime girlfriend, singer Sierra Deaton, to be his wife.

The couple kept the proposal, which took place earlier this year, a secret until now.

Luke captioned the sentimental post: "With shaking hands but a full heart I proposed to my best friend earlier this year.

I love you Thao and I can’t imagine my life without you @sierradeaton (sic)"

The former 'X Factor US' star - who competed as Alex & Sierra with ex-partner Alex Kinsey - gushed that she "can't wait forever" with the 24-year-old hunk.

She added to her own profile: "It was fun keeping this a secret for so long, but my heart is nearly bursting at the seams. can't wait for forever @lukehemmings. (sic)"

The pair first started dating in February 2018.

Luke previously revealed that the pop-rock band's 2020 song 'Not In The Same Way' is about his partner, who he gushed is a "beautiful human".

He said: "Well, it's about me and my current girlfriend, and she's incredible. Maybe Ashton was there as well, maybe he was writing about someone else, but that's where I was writing about from.

"She's a beautiful human. It could be painted in such a... when the worst moments are written on paper, it doesn't look very good. It was very important that it was the other side of it, because a lot of the mistakes are mine, you know?"