Riccardo Agostini will make his GP3 debut with Hilmer at this weekend's Red Bull Ring event.
The 20-year-old Italian began the 2014 season driving for Eurointernational in European Formula 3.
He will take the Hilmer seat that Formula Renault 3.5 racer Beitske Visser occupied for the GP3 opener at Barcelona last month, and could continue in the series subject to results in the Austrian Grand Prix support event.
"It's a unique opportunity to be under the spotlight, even if the level of the series is very high," said Agostini.
"I will be in the front line again and I can't wait for it."
Agostini won the 2012 Italian Formula 3 title before partial campaigns in Auto GP and FR3.5 last year.
Team boss Franz Hilmer said he had been watching Agostini for a while.
"We followed Riccardo's career closely in the past and he was already testing with our GP2 team in the pre-season test days last year," he said.
"He is a very talented young race driver and we are convinced that he can be a frontrunner."
Agostini's arrival is the second Hilmer GP3 driver change announced for this weekend, with FR3.5 podium finisher Nikolay Martsenko taking the place of fellow Russian Ivan Taranov in the team's #17 car.