Reigning British Formula 3 champion Jaime Alguersuari will take over Red Bull reserve duties from Brendon Hartley from the German Grand Prix.
The 19-year-old Spaniard will be available to deputise if any of the Red Bull or Toro Rosso drivers are sidelined, and will work with the teams at Formula 1 weekends.
A team statement said that the move was part of Red Bull's development plan for the duo, who are both part of its young driver scheme.
"With the ban on in-season testing, Red Bull views the appointment of a third driver as part of its Young Driver training programme, allowing a driver to follow all activities of the F1 teams over a race weekend, attending technical briefings and listening in on the pit to car radio conversations," said a team statement.
"Having benefited from this experience over the first half of the year, Hartley will now concentrate on his own racing programme in the European F3 series."
Alguersuari, who was teamed with Harley at Carlin Motorsport in British F3 last season, is currently competing in the Formula Renault 3.5 Series. Although his F1 mileage has so far been limited to straightline tests and demonstration events, he already has a superlicence thanks to his F3 title.
Hartley did not receive his superlicence in time for the start of the F1 season, so retired Red Bull driver David Coulthard was the official reserve until the Spanish GP.