Latifi had a development role with Force India this season that involved three days of testing with the team and some Friday practice running.
The Canadian, who finished ninth in F2 after struggling to adapt to the new car, will have an enhanced role with Williams.
His switch to the team was expected following Force India’s reincarnation as a new entry backed by a consortium led by Lance Stroll’s father Lawrence.
Latifi will drive the team’s new FW42 on one day in pre-season testing and three in-season test days – one official day and two as part of Pirelli’s tyre testing programme – as well as six FP1 seasons.
“I’m thrilled to be joining an iconic team like Williams as a reserve driver for 2019,” said Latifi.
“It’s a fantastic opportunity to continue my F1 development, and to build my on-track experience with more FP1 sessions and the rookie and Pirelli tests.
“As well as this, a big part of my role will be simulator driving and helping to support the development of the new car.
“I look forward to spending time in the factory at Grove, immersing myself within the team and assisting wherever I can.
“This is an amazing opportunity that I’ve been given and I’m excited to get started.”
Latifi’s fifth place in the F2 standings in 2017 boosted his superlicence prospects but he only added four points to his tally by finishing ninth this season.
He now has 24 points and will need to finish in the top five next year in F2, where he is expected to contest a fourth season, to clear the 40-point threshold for a superlicence.
“Nicholas has been racing successfully in the junior formula, he has the racing pedigree that we’re looking for and he is incredibly intelligent and diligent,” said Williams deputy team principal Claire Williams.
“We are certain he will be a great fit for the team and we look forward to working with Nicholas next season.”