The 24-year-old Dutchman - who has been the British-based team"s test and reserve driver this season - will race in China, Japan and Brazil.
Doornbos said: "It's fantastic to know that I'm going to be racing again, I'm really looking forward to it.
"Friday testing and helping the team this year has been great experience, but to get back out on to the race track on Sunday is what it's all about.
Formula 1
Alonso slams Schumacher
12/09/2006 AT 09:42
"I've got to know the guys at Red Bull Racing really well this season, so I'll be working with them all closely to do the best job I can for the team and myself."
Doornbos made his Grand Prix debut last year for Minardi, racing eight times and taking a best finish of 13th in Turkey and Belgium.
Red Bull team boss Christian Horner explained that Doornbos was the logical replacement for Klien - who has been released from his contract after rejecting a Red Bull-backed move to the US-based ChampCar World Series next year.
Horner said: "Further to Christian's recent decision, we have released him to focus on the next, stage of his career.
"Having done a solid job for us in the third car this season and having previously completed in eight Grands Prix, Robert is the logical replacement.
"He will start preparing for his Red Bull Racing debut at the team's test in Jerez this week."
Formula 1
Ferrari to offer Schumacher test role
12/09/2006 AT 09:14
Formula 1
Euro Spout: Schumacher
11/09/2006 AT 10:56