Williams have retained Felipe Massa and Valtteri Bottas for a third season.
The announcement on the eve of the Italian Grand Prix ends speculation Jenson Button, whose future at McLaren next season remains in limbo, could link up with the team where he started his Formula One career in 2000.
Bottas had been mooted to drive for Ferrari in 2016, but that link was dashed after the Italian team surprisingly retained Kimi Raikkonen for next season.
Bottas, who joined the Williams race set-up in 2012 before making his grand prix bow a year later, said: "I am really pleased to be staying with the team for a fourth F1 race season.
"We have been getting stronger as a team and I look forward to what next year will bring, as I believe we can achieve even stronger results than what we have seen so far.
"I can't wait for 2016, however I am still fully focused on this season as we still have plenty of challenges to achieve."
Massa joined Williams after nine seasons at Ferrari. Ahead of Sunday's race in Monza, the Brazilian, who narrowly missed out to Lewis Hamilton in the championship in 2008, is fifth in the drivers' standings with 82 points.
He said: "I am very happy to be a part of the Williams family. I am in my second year here and am so pleased to be staying as we are working very well together as a team.
"They respect me a lot and the results are there, and it has been so nice to be part of the big turnaround the team has seen since the end of 2013. I can't wait to carry on and continue pushing even more for the team and for myself."
Source: PA
