Max Verstappen will join Antonelli on the front row with Mercedes team-mate and title rival George Russell right behind.
Piastri's McLaren team-mate, world champion Lando Norris, was third but will lose 10 places for Sunday's race owing to a penalty, a drop that meant Mercedes' George Russell will share the second row with Ferrari's Charles Leclerc.
Piastri, who won last year's Belgian Grand Prix, will hope for an easier ride in Sunday's race proper after a difficult qualifying session.
"It was a trickier day for us. We have struggled to find the right balance with the car and extract the maximum lap time," the 25-year-old Melburnian said.
"Our final lap in Q3 was a step in the right direction, but it's clear we haven't had an easy time. We had to use an extra set of new tyres in Q2 just to be safe, which shows how challenging the session was.
"Looking ahead to the race, it's difficult to predict our exact pace since our running has been limited. The pack around us is incredibly tight, however a race here is always eventful, especially on the first lap, so we'll be looking to make the most of any opportunities that arise.
The pole was the 19-year-old Italian Antonelli's sixth in 10 rounds and continued Mercedes' run of being quickest in every qualifying round so far this season.
"It's great to be on pole," Antonelli said. "It was not a very straightforward session, the track changed a lot. But we were able to improve lap by lap and to bring home pole, which was nice.
"But tomorrow is another day and obviously I have Max starting next to me so it's going to be important to get a good start and then be ahead into Turn Five," he warned.
Dutchman Verstappen had set the fastest time after smart coordination with Rteammate Isack Hadjar to give him the strongest possible slipstream on his lap.
And Verstappen said Hadjar's help was the difference between being second and sixth but admitted his Red Bull still can't keep up with Antonelli's Mercedes in race conditions.
"He did amazing. I initially thought 'Oh my god, it's too close.' Actually it worked out well to the last corner. It was close but I trusted him," Verstappen said of Hadjar.
"The gap in qualifying even with a massive tow is still three-tenths, so I don't really expect to race (Mercedes) tomorrow," Verstappen added.
Ferrari's Lewis Hamilton, chasing a record sixth Belgian GP win, will start fifth ahead of McLaren's Piastri having crashed in final practice and then relying on his mechanics to get his car ready to start qualifying.
Racing Bulls' British rookie Arvid Lindblad was seventh in the team's only upgraded car.
Audi's Gabriel Bortoleto qualified eighth with Racing Bulls' Liam Lawson set to start ninth, despite failing to make the final phase, and Alpine's Pierre Gasly 10th.
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