The 'Lose You to Love Me' hitmaker has heaped praised on her best friend and thanked her for "teaching" her so much and said she would be "on her side for life".
Taking to her Instagram story, she wrote: "My ride or die. I would die for this one. Thank you for forever being by my side. You have taught me so much, walked through all of this with me, STAYED and you remind me to be a better, human being. I'm on your side for life."
Meanwhile, Taylor previously insisted she is "so proud" of Selena.
She said: "It's the best thing she's ever done [her new music]. She came over, she played me the video. I'm so proud of her. She's been through so much. I've watched so much happen in her life and had a front row seat to so much, and I'm so proud of her. She is such a revelation because now she's making her best stuff. I am 100 percent convinced this is the best thing she's done so far ...
"You know, I think no one can say that better than she can, but just from my perspective as her friend, I've just never been more proud. The things that she's overcome, the situations that she's risen above. It's been a long time that we've been friends, and she's the best. She's just the best. I'm really excited because when somebody has had great life experience, has had really tough things they've had to go through, and they can process that and make art that's going to help other people, that's what kind of song this is. I'm stoked."