Brendon Hartley, a 19-year-old New Zealander, will be reserve driver for both Red Bull teams in 2009.
The news was confirmed on Monday as Toro Rosso team boss Franz Tost spoke with reporters at the Barcelona test.
"We (Toro Rosso and Red Bull Racing) will share the third driver. It is Brendon Hartley, who is the driver from the Red Bull driver pool," the Austrian said.
Japanese grand prix veteran Takuma Sato, previously the favourite for the reserve role and also a Toro Rosso race seat, last week revealed he was disappointed an agreement with Red Bull could not be reached for this season.
Hartley, who contested British F3 last year, recently qualified for a mandatory FIA superlicense by completing the requisite mileage at a winter test.