This Rocker Has Been Writing All Of Your Favorite Pop Songs

Meet the guy behind Justin Bieber, Selena Gomez and Fifth Harmony's hit songs.

You may recognize Justin Tranter as the lead singer of Semi Precious Weapons or as one of Lady Gaga's BFFs, but what you probably don't know is that he's co-written so many of your favorite songs over the past year.

From Selena Gomez to Justin Bieber to Fifth Harmony, Justin has been a part of the biggest hits in pop music, and it's about to get even bigger as he is working on Britney Spears' upcoming ninth album. The pop star shared a photo of Justin, along with his frequent writing partner, Julia Michaels, saying they were "Working hard and hardly working" on her upcoming LP.

So, to get you up to speed on who exactly Justin is and what he's done, here are 10 songs you didn't know that Justin helped co-write.

"Sorry" - Justin Bieber

Tranter teamed up with Bieber for his second single off Purpose, saying that the song is simply about making difficult apologies: "Truly saying sorry is never easy to do, and when you are you just hope it’s not too late."

"Good for You" - Selena Gomez ft. A$AP Rocky

Justin teamed up with Selena for the majority of her tracks on Revival including "Hands To Myself," "Revival" and "Body Heat."

"Used To Love You" - Gwen Stefani

Gwen's latest single is raw, emotional and extremely heartbreaking, and she turned to Justin to help her bring it to life. And if you like what you hear, then good news -- Justin wrote in an Instagram post along with Gwen that he's "Making so many song babies with this is awesome!"

"Love Myself" - Hailee Steinfeld

Not only did Justin help co-write Hailee's debut single, he also lent his writing skills to her upcoming EP, Haiz (Nov. 13).

"Cake By The Ocean" - DNCE

This collaboration actually makes total sense. DNCE member Cole Whittle is actually the bass player in Semi Precious Weapons, so he and Justin go way back.

"Like Mariah" - Fifth Harmony ft. Tyga

Justin teamed up to help 5H release not only my favorite, but Mariah Carey's favorite track off their debut album, Reflection. In fact, it seems like they all enjoyed working with each other so much that Justin is now helping the girls out on their next album, recently sharing a photo of himself with the girls in the studio.

"Touch" - Pia Mia

Justin was part of Pia's brand new single, "Touch," an uptempo and almost exotic, tropical track.

"Nostalgic" - Kelly Clarkson

Justin had the idea for "Nostalgic" long before it got in Kelly's hands. In fact, he actually the sang the demo that Kelly heard, and she loved it, so she recorded it and added the song to her album, Piece By Piece. In a recent interview, Justin said that he started crying in a meeting with executives when he first heard Kelly's version.

"Run" - Nicole Scherzinger

Justin wrote this track with Michaels, who had the original idea for the song, but the two worked together to make it a track about telling a person you are about to date, "I'm an a--hole -- run away from me."

"Centuries" - Fall Out Boy

Justin's first "big song" came from "Centuries," which he said was actually meant for a rapper to rap the verses and a female singer to to take on the chorus. However, FOB heard it, wanted to do it and it became a huge hit for not only Justin, but the band.