The 25-year-old tested with the Grove-based squad at the end of the 2019 season in Abu Dhabi, driving the FW42 during the two days of running.
Williams confirmed the signing of Nissany at a press conference in Tel Aviv, where Nissany was born.
As part of his duties, Williams has confirmed that Nissany will take part in three free practice one sessions throughout the year, while also taking part in one rookie test day.
“I am thrilled to become Official Test Driver for ROKiT Williams Racing," Nissany said. "This is also a landmark for motorsport in Israel.
"When I tested for the team in Abu Dhabi in December, I instantly felt comfortable in the car and part of the team.
"The experience that I will gain this year, immersing myself with Williams will prove invaluable and I’m looking forward to getting started, both on and off the track.”
Nissany will also handle simulator duties as Williams looks to bounce back from a disastrous 2019 season that saw it consistently compete at the rear of the field.
Williams also announced that Nissany will race in the FIA Formula 2 championship this year, having raced most of the 2018 campaign.
“It is a pleasure to welcome Roy Nissany to ROKiT Williams Racing in the role of our Official Test Driver," said Williams deputy team principal Claire Williams.
"Roy demonstrated his capabilities driving in the post-season Abu Dhabi test last year and we were extremely impressed with what he could do in a short space of time. He is a hard-working individual who we are excited to be working with this year.”
Bahrain International Circuit - Winter testing