Teddy Swims & Tiago PZK Bond Backstage Giving Each Other Tattoos

Teddy Swims and Tiago PZK took their friendship to a new, permanent level ... giving each other tattoos backstage at Lollapalooza Argentina!!!

Hours before gracing the Lollapalooza stage on March 22, they both surprised 2,500 fans with a pop-up performance at Parque Rivadavia where a fan gave Teddy a Boca Juniors scarf souvenir.

After mesmerizing the crowd, they both whipped out the tattoo guns backstage and impressively left their mark -- on the other person.

We got the proof ... Teddy added a cutesy bear with his John Hancock to Tiago's previously bare leg, and the megastar Argentine reggaeton-rapper returned the favor, ingraining the name "Gotti" into Teddy's thigh, while affectionately dotting the "i" with a heart!!!

Later at Lollapalooza, Teddy and Tiago both graced the stage together to KO their hit collabs "Sometimes" and Tiago's version of Teddy's classic track "The Door" in front of a crowd of the purported 400,000 raging fans in attendance!!!

If the tats weren't proof enough of their bond, Teddy and Tiago have also become strong collaborators.

The Warner Music labelmates connection has blossomed into an international banger ... "Sometimes" was released just a couple of weeks ago and already has 4 million streams on Spotify alone!!!