Piastri has made a name for himself in recent years, winning the FIA Formula 3 championship last year and leading the 2021 Formula 2 title race with just two events remaining.
There were hopes that the 20-year-old Victorian could make an immediate step up to the pinnacle for next season.
However, being an Alpine Academy member, his path to the top-flight with the French manufacturer was blocked as Fernando Alonso's renewal to the end of 2022 was confirmed and Esteban Ocon was re-signed to a three-year deal.
Piastri had been considered an outside chance of nabbing the last remaining vacancy at Alfa Romeo, but has now opted to remain with Alpine, for whom he will complete “an extensive testing programme” with a view to a future race seat.
“I'm super excited to be joining Alpine F1 Team as reserve driver,” said Piastri.
“I'm looking forward to being much more involved with the team and contributing to its intended success next season.
“The reserve driver role is the next step towards my aim for a race seat in 2023, which is very exciting.
“I've proved myself in the junior formulas over the last couple of years and feel like I'm ready for Formula 1 now; along with the trackside experience at race weekends, we will put together a substantial test programme in order to keep developing myself to grow even more prepared for a race seat.
“I'm very thankful to Alpine for their support. We've enjoyed two very successful seasons together in the Academy and I'm grateful for the faith they've put in me for this next step with an eye on a bigger future.
“My focus now is finishing the Formula 2 Championship in the best possible way with Prema and I'm looking forward to being back in the car and pushing hard on track.”
Should he go on to convert his 36-point advantage into a championship, he would be ineligible to compete in F2 next year in any case.
He will replace current Alpine reserve Daniil Kvyat, who is tipped for a switch to the World Endurance Championship.
Alpine CEO Laurent Rossi offered a glowing endorsement of Piastri as he congratulated him on the appointment.
“Oscar's natural talent is clear to see, so we're very proud and privileged to have him as part of our team as our reserve driver from next year,” said Rossi.
“Not only does Oscar have the on-track skills, as demonstrated over the last couple of years in the junior categories, but also the maturity and composure that makes him really stand out from the rest.
“At Enstone he has been a real asset to the team as he has aided the simulator and testing programme and I am certain he will continue to do so in his new role.
“The next step will be to consistently attend Grands Prix to fully integrate with the race team, learn what is expected of an F1 driver and be ready for when the opportunity comes.
“In addition, we look forward to having Oscar's input and expertise in driving the team forwards next year.
“Oscar's graduation highlights the success of our Alpine Academy and underlines its strength as one of the best young driver programmes in motorsport. We look forward to seeing our young talents continue to blossom through the years to come.”
Alpine holds fifth in the constructors' championship, with Ocon having delivered victory at the Hungarian Grand Prix at the start of August.