Ricciardo, a winner of two races this year, has been linked to Renault in media speculation.
“Daniel Ricciardo has advised us that he is to leave the team at the end of the 2018 season,” Red Bull said in a statement without saying where he was headed.
The surprise announcement will ripple through the driver market, with other teams set to shake up their line-ups as a consequence.
He has also been linked to McLaren but Renault, as a manufacturer team in fourth place in the championship, looks like a more desirable option.
There was no immediate comment from the French team.
“We fully respect Daniel’s decision to leave Aston Martin Red Bull Racing and we wish him all the best in his future,” Red Bull team boss Christian Horner said.
“We would like to thank him for his dedication and the role he has played since joining the Team in 2014, the highlights of course being the seven wins and the 29 podiums he has achieved so far with us.
“We will now continue to evaluate the numerous options available to us before deciding on which driver partners Max Verstappen for the 2019 season.”
Ricciardo had been out of contract at the end of the year and was assessing his options, but any hope of joining champion Mercedes disappeared when it retained titleholder Lewis Hamilton and Finland’s Valtteri Bottas.