Roy Nissany will continue as Williams test driver for the 2021 season.
The 26-year-old Israeli will have three outings in practice sessions at Grands Prix, plus one day of pre-season testing as part of his deal.
It means race drivers George Russell and Nicholas Latifi will have one day in the car each before the season-opening Bahrain GP on 28 March.
F1 pre-season testing has been reduced to just three days in 2021, also in Bahrain in March, to limit costs.
Last season, Nissany took part in free practice sessions for Williams at the Spanish, Italian and Bahrain Grands Prix.
"I am truly honoured to continue being part of Williams," he said.
"Since we started, we have made huge progress across many areas - on top of the great FP1's, we had a lot of productive work behind the scenes.
"While dedicating as much as I can to the team, I enhanced my skills and my capabilities as a driver. I am very keen to continue this form in 2021."
Team principal Simon Roberts added: "We were very happy with the contributions Roy made both through his work on-track and in the simulator at Grove, which all aided the lap time gains we were able to make with the FW43.
"We also enjoyed seeing Roy grow as a driver, and we have no doubt that he will continue to go from strength to strength this year."
Mercedes, meanwhile, have signed Dane Frederik Vesti to their junior driver programme.
The 19-year-old finished fourth in the FIA Formula 3 championship last year and will continue in the series in 2021.
As a Mercedes junior, Vesti follows in the footsteps of many significant names, including current F1 drivers George Russell, of Williams, and Esteban Ocon of Renault.