Two-time world champion Alonso announced on Tuesday that he had decided to leave McLaren and “move on” from F1 at the end of the season.
“I’m delighted to finally be able to confirm that I’ll be a McLaren driver from the 2019 season,” Sainz said on the team’s official website.
“It’s something I’ve been looking at for a while and I’m very excited about this next chapter in my career.
“I’ve been a McLaren fan for as long as I can remember. It’s a great name in the sport with an incredible heritage, and the list of drivers that have raced for McLaren over the years are among the heroes of Formula 1.”
Fernando Alonso has announced that he will leave McLaren and “move on” from Formula One at the end of the season.
Alonso, a two-time world champion, is currently racing in his 17th season in F1 and has promised to finish the 2018 championship with “more commitment and passion than ever”.
The 37-year-old Spaniard says “new exciting challenges are around the corner” and is expected to make a full-time switch to the Indy Car series.
Alonso said in a statement: “After 17 wonderful years in this amazing sport, it’s time for me to make a change and move on. I have enjoyed every single minute of those incredible seasons and I cannot thank enough the people who have contributed to make them all so special.
“There are still several grands prix to go this season, and I will take part in them with more commitment and passion than ever.
“Let’s see what the future brings; new exciting challenges are around the corner. I’m having one of the happiest times ever in my life but I need to go on exploring new adventures.”
Lewis Hamilton holds a 24-point championship lead heading into the second half of the season.
The Mercedes scored back-to-back wins in Germany and Hungary to stretch his advantage ahead of rival Sebastian Vettel with nine races remaining.
Here, we assess the qualifying head-to-head at each team.
Mercedes: Lewis Hamilton 7-5 Valtteri Bottas
Ferrari: Sebastian Vettel 10-2 Kimi Raikkonen
Red Bull: Daniel Ricciardo 2-9 Max Verstappen*
Renault: Nico Hulkenberg 7-5 Carlos Sainz
Haas: Kevin Magnussen 8-3 Romain Grosjean*
Force India: Sergio Perez 3-9 Esteban Ocon
McLaren: Fernando Alonso 12-0 Stoffel Vandoorne
Toro Rosso: Pierre Gasly 7-3 Brendon Hartley**
Sauber: Marcus Ericsson 3-9 Charles Leclerc
Williams: Lance Stroll 5-7 Sergey Sirotkin
* Missed 1 Qualifying session
**Missed 2 Qualifying sessions