Justin Bieber and Selena Gomez release duet!

Justin Bieber and Selena Gomez have released a song about their relationship.

The former couple - who had a tumultuous four year relationship - have dropped a surprise new collaboration called Strong in which they appear to discuss their romance.

READ: Selena Gomez 'I Was Angry At Justin Bieber'

READ: Selena Gomez 'I've been having Chemotherapy'

READ: Justin Bieber Says He Was 'Married' To Selena Gomez

Justin sings: "You're the only one I'm really missing. Nobody else would be forgiving, cause of all the love that you give me."

And Selena responds: "Wishing I was better at this damn thing, cause it's just been too long and I can't change."

Selena, 23, recently insisted that although she was heartbroken about the end of her relationship with Justin, 21, the pair have now moved on and gotten closure.

The duo have reportedly been planning the collaboration for some time as they felt it would be "therapeutic".

A source told HollywoodLife.com: "[Selena and Justin] didn't record this together; they laid their tracks separately but were both very involved with the song's creation and writing. They weren't the main writers, but they will be credited as writers among others. This has been planned for months and they have been working on it at the same time they were working on their own albums.

"They are still very tight and [Justin] felt that it would be very therapeutic if they worked together. Their past could lead into some amazing future business decisions because they both know how each other tick."