The 27-year-old singer - who hasn't confirmed who or what the tune is about, or exactly when it was written - has admitted she has a batch of tracks recorded during her 'Rare' sessions which didn't make the cut for her latest album.
Appearing on 'The Tonight Show Starring Jimmy Fallon', Selena was asked if she had any unreleased but finished songs she wanted to drop and teased: "Maybe there's a few things, yeah.
"There are a few other songs that I can't help but want to exist, so I can't really tell when - but one of my favourite tracks is called 'Boyfriend' and I can't wait for people to hear that one."
The 'Lose You to Love Me' hitmaker released the new collection - her first album for five years, after 2015's 'Revival' - on January 10, and she has admitted the record was "a nightmare to deal with".
She recently explained: "Honestly, this album was kind of a nightmare to deal with, but in the best way possible.
"I thought that I was so ready like two years ago ... I had collected all of these sessions, beautiful sessions, that I don't even remember half of them because it evolved each year."
Selena has battled some health issues in recent years, including undergoing a kidney transplant as part of her battle with lupus in 2017, and she admitted "very drastic" changes in her life led to her album going in different directions to how she initially intended.
She said: "And then something would happen to me and it would be very drastic, so that it would take me somewhere else.
"And then all of a sudden I found this healing, and I saw something bigger than what I thought that I was. And I think that also added the whole end of the album."