The Aussie, looking bright around Barcelona's Circuit de Catalunya, was quickest after two laps of Q3, the final run at qualifying, clocking 1 minute 15.176 seconds.
But then Ferrari's Charles Leclerc - under pressure after a mixed start to the season - suffered an unusual snap at Turn 4 and overcorrected, resulting in a trip through the gravel before he piled into the barrier.
Once the action resumed, with eight minutes remaining, Mercedes pair George Russell and Kimi Antonelli, along with the other Ferrari driver, Lewis Hamilton, took control.
Russell will start on pole on Sunday, in front of Hamilton. Italy's Antonelli, who has a 66-point lead at the top of the standings over seven-time world champion Hamilton, qualified third, with Piastri's McLaren teammate, Lando Norris, fourth.
Piastri, who won the Spanish Grand Prix from pole last year, will start from seventh position on the grid. "It was a good lap until the last sector," he said.
"It was incredibly tight today, with such a small gap from third down to us in P7. We lost more than expected in the final sector, so we need to review why because the lap up to then felt good."
Piastri and Norris could have endured a worse fate. The McLaren pair were at risk at the end of Q2 with world champion Norris eighth and Piastri down in 10th but they improved as the afternoon developed.
Mercedes have taken pole in all seven races so far in 2026, winning them all, and Saturday was Russell's third of the season to 19-year-old Antonelli's four, a morale boost for the Briton as he seeks to bounce back from two races without scoring.
"Nice to feel in the groove again," said Russell over the team radio after lapping 0.064 of a second quicker than compatriot Hamilton for a 10th career pole ahead of what will be his 100th grand prix for Mercedes.
"It's been a great weekend so far. I feel like my old self again where every lap I'm doing the job, always fighting in those top positions," he added later.
Hamilton said: "This is the closest I've been to the front qualifying right? So I will give it a go.
"I think clearly these guys (Mercedes) just seem to have extra in the pocket, every time we bring an upgrade they are still ahead so we've got some work ahead of us. It's great for us, this is the closest we've been pace-wise."
It was a disappointing session for Fernando Alonso, who had the slowest time in his Aston Martin. He will start from last place in what the 44-year-old former champion said will likely be his last race in Barcelona.
The two Cadillacs, driven by Sergio Pérez and Valtteri Bottas, failed to advance from the first qualifying session that culled the slowest six cars.
With agencies
7°C