Gomez, 27, and Swift, 30, discussed their sister-like bond in the February issue of WSJ. Magazine.
"We clicked instantly and, man, that was my girl," Gomez said.
"There has always been this quality of sisterhood, and I don't say that in a basic way," Swift added. "I know from when I met her I would always have her back. In my life, I have the ability to forgive people who have hurt me. But I don't know if I can forgive someone who hurts her."
Gomez was counted among Swift's high-profile "squad" of celebrity friends, but said her friendship with the "Bad Blood" singer was cemented in private.
"There's so much of my friendship with Taylor that people don't know about because we don't necessarily feel the need to post about everything we do," Gomez said.
"She's showed up for me in ways that I would have never expected. Flown in because I was hurt and was going through something. Stuff that was going on with my family," she added of Swift. "It's been proven year after year and in every moment of my life that she is one of my best friends in the world. We don't agree on everything, but we respect each other with everything."
Gomez previously said in an interview with KISS FM UK that Swift was one of the first to hear her forthcoming album, Rare. She recalled how Swift cried after hearing her songs "Lose You to Love Me" and "Look at Her Now."
"It was one of the coolest experiences because I've been friends with her for over a decade and love her family as well," Gomez said of playing the songs for Swift and the singer's family.
"[Taylor] played it and her and her mom just started crying, tears and tears," Gomez said. "It wasn't just about how great the song was, which is a lot coming from her, it was just that they had been on that journey with me, intimately."
Gomez will release Rare on Friday. The album is Gomez's first since Revival, released in October 2015.