But Selena and Demi go way back -- all the way back to 2002, as two seven-year-old girls on Barney and Friends. Take a look below at a timeline of the pop star duo's friendship through the years.
Barney and Friends
There's the era of The Mickey Mouse Club, then there's Demi Lovato and Selena Gomez on Barney and Friends. They appeared on the show from 2002-04 -- Selena playing a character named Gianna, and Demi one named Angela.
In the clip below, Gomez can be seen playing patty cake, with Lovato coming in later (dressed in pink) to chat with Barney.
Wizards of Waverly Place and Sonny With a Chance
As the young entertainers grew, they graduated to Disney. Gomez starred in Wizards of Waverly Place from 2007 to 2012 as Alex Russo -- one of three kids in a family full of wizards.
Lovato's Disney show was Sonny with a Chance, which ran from 2009 to 2011. Lovato played the title character, Sonny Munroe, a child star in the making. Lovato also starred alongside the Jonas Brothers in the Camp Rock franchise.
You can still binge watch Wizards here, and clips from Sonny here.
All of the vlogs
?Demi and Selena's first vlog arrived to Lovato's official YouTube account on Jan. 29, 2008. They both greet the camera and thank everyone for supporting them, before Gomez reveals they are about to go to Puerto Rico together to shoot a Disney Channel original movie (which turned out to be Princess Protection Program).
"And this is my best friend," Lovato says, pointing to Gomez, "in the entire world."
"And this is my best friend ever since we were seven years old," Gomez says.
"On Barney!" Lovato adds.
If you go to Lovato's YouTube channel and scroll all the way to the bottom, you can OD on nostalgia by watching all of their vlogs together.
The 2009 People spread: "We enjoy doing everything together."
In 2009, People ran a spread titled "Demi & Selena: Inside Their Friendship!"
The photo spread featured the two in the middle of a pillow fight, with a Q&A included. When asked what their favorite thing to do together is, Gomez answered, "We enjoy doing everything together. But our favorite is to chill at each other's houses."
To read the full flashback Q&A, click here.
Reportedly, a wedge was injected into the "Delena" friendship when Selena started hanging out more and more with Taylor Swift. People took notice when, in 2010, a fan asked Lovato outside her hotel in London how Selena was doing and Lovato simply quipped, "Ask Taylor."
This comment could have been wildly blown out of proportion but other signs of the two drifting apart were evident. Also in 2010, Lovato spoke to Girl's Life about Gomez. "We're both busy, and I wish the best for her," she said. "True friends don't let their friends or family be mean to you. If you can't trust somebody, you can't be friends with them."
Reconciliation during Demi's rehab stint in 2011
In 2011, Lovato entered rehab for substance abuse. While in there, Lovato revealed that Gomez checked in on her. "[Selena] called me crying when I was in there and was just so worried," Lovato said. "I was like, 'Look, I should be the one crying, it's all good!' But she was so worried and ever since then, we've gotten closer."
Supporting each other's music again
In 2013, Lovato tweeted support for Gomez's new single "Come & Get It," saying, "Umm..obsessed with Come & Get It - @selenagomez. So happy for you lady. #LiterallyOnRepeat!!!"
Also in 2013, Lovato visited Live From MTV and delved into the state of her relationship with Gomez.
“We’ve been through a lot together, we’ve had periods of times we here we grew apart and we just didn’t really talk,” she said. “Now we are in a place where I think we realized that life is so short and that when you have people in your life that love you so much then, you should just always be around them. She’s going to be a big sister, and I want to be around for that. Everything about her -- she has a great soul, she’s talented, she’s beautiful and she will always be really, really close to my heart.”
Later in the year, Gomez returned the sentiment on Instagram: