Piastri scorched around the Interlagos circuit late in the three-part session to pip McLaren team-mate Lando Norris to top spot by 0.029s.
The Australian had remained on track following an initial flying lap, cooling his tyres before a second effort as the chequered flag fell.
“Very, very happy,” Piastri said of his performance.
“Always nice to be on pole. Hasn’t happened enough this year, I would say, so nice to get the first one.
“Felt like it was a pretty good lap, car felt pretty good as well, so very happy with it.”
McLaren opted for an alternate approach to the final segment of the session, sending both Piastri and Norris out in the early moments.
That afforded both a clear run, Norris recording a 1:08.928s.
Without time to box in the short SQ3 session, both McLaren drivers remained on track, Piastri even touring through the pit lane to help cool his tyres.
On his second lap, Norris was unable to improve and aborted the effort, while Piastri made strong gains despite his (comparatively) used tyres to snatch pole.
“I’m not sure really,” he confessed to Sky Sports when asked where the lap came from.
“The very first lap of the session in SQ1, I put in a really good lap, and then didn’t do another one at the end of SQ1.
“I think the track was just really moving on really quickly – I think it was like a second and a half faster by the end.
“So I was catching up a little bit through the rest of the runs, and the last lap on softs, it hung on for a second lap which was nice and managed to put it on pole.”
Track evolution was sharp on the newly resurfaced venue.
However, while offering improved grip it has come under fire for the bumps it has introduced.
“I think seeing the end of the straight is the first objective,” Piastri said when asked what his main concern was on the new surface.
“It’s bad. It’s really, really bumpy.
“I think everyone’s had to change a lot of things on the car to make them somewhat comfortable.
“The grip is actually quite good, it’s just trying to keep all four wheels on the ground at once is not that easy.”
The 24-lap Sprint begins at 11:00 on Saturday (local time).
