The news clarifies the 'drama' in Austria at the weekend, when the 22-year-old driver said that it is "unlikely" he will stay with Red Bull's junior team for a fourth consecutive season.
Red Bull chiefs bit back hard at the Spaniard, although Sainz insisted: "It was not even half the drama that it seemed.
"The subject is closed. My bosses trust me, they have renewed with me, so I don't see the problem."
Dr Helmut Marko, Red Bull's driver manager, indicated that the drama was created by Sainz's apparent lack of commitment to Toro Rosso for 2018, even though the energy drink company had officially triggered the 'option' in his contract mere days ago.
"We sent him a letter saying that we take up the option," Marko is quoted as saying by newspaper Diario Sport.
"As far as I know - and since I have been in Red Bull it has been like this - it is not the driver who decides what happens with the contract."
The 2017 season continues on Sunday with the British Grand Prix.