Ricciardo had already announced he is moving to Renault, leaving a vacancy alongside 20-year-old Dutch driver Max Verstappen.
The French Formula One driver Pierre Gasly will graduate from Toro Rosso to Red Bull Racing as replacement for departing Australian Daniel Ricciardo next year.
"We look forward to Pierre bringing his speed, skill and attitude to the team in 2019".
Gasly won the GP2 (now Formula Two) title in 2016 and made his F1 debut with Toro Rosso in Malaysia late a year ago before starting his first full season in March.
Making several eye-catching performances in the junior Red Bull team - Toro Rosso - Gasly has shown that he is worthy of being called up.
He finished an impressive fourth in Bahrain and has two further top 10 finishes to his credit from the first half of the campaign.
By signing Gasly, Red Bull Racing will likely have the youngest lineup on the Formula 1 grid next season.
"It has been my goal to race for this team since I joined the Red Bull Junior Driver Programme in 2013, and this incredible opportunity is another step forward in my ambition to win Grands Prix and compete for world championships", said Gasly. I'm a really competitive guy and when I do something it's always to fight for the best and top positions.
That victory proved the catalyst for a title tilt and he edged Prema team-mate and now Ferrari reserve driver Antonio Giovinazzi, though was left without an F1 seat as Red Bull kept Daniil Kvyat at Toro Rosso.
"As hugely exciting a moment as this is, I am fully aware of the challenge this special opportunity offers me and the expectations that faces any driver at Aston Martin Red Bull Racing", continued Gasly. I'm looking forward to rewarding the faith that Dietrich Mateschitz, Christian Horner and Dr Helmut Marko have placed in me. I wish to thank Franz Tost and everyone at Toro Rosso and the factory in Faenza for giving me the golden opportunity of a drive in Formula One and their unbelievable commitment and support, which I know will continue for the rest of this season.
The team is yet to confirm any drivers for next season, but Hartley's retention would give them some continuity for 2019.
"While our immediate focus remains fully on achieving our best result in the 2018 World Championship, we look forward to competing in 2019 with Max and Pierre at the wheel of Aston Martin Red Bull Racing". The team has been linked with Lando Norris and Stoffel Vandoorne in recent weeks.