Williams will use Cosworth engines next season with their 2010 driver line-up to be announced this week, the Formula One team said.
Williams said in a statement that they had signed a long-term agreement with Cosworth, who last powered them in 2006, to replace Toyota.
"Cosworth have impressed us with the quality of their technical team and convinced us that their programme will deliver a competitive Formula One engine," said team owner Frank Williams.
"In addition, like Williams, Cosworth is an independent company whose owners share our passion for racing and engineering.
"We believe that we will develop not only a competitive racing car for 2010 but also a long-term partnership that can take on the best in F1."
Williams, whose team are former champions but last won in 2004, was quoted in Gulf News as saying the drivers would be announced tomorrow.
Germany's Nico Rosberg has already said he was leaving the team, with Williams widely expected to name an all-new pairing of veteran Rubens Barrichello and Germany's GP2 champion Nico Hulkenberg.
Japan's Kazuki Nakajima, Rosberg's team-mate this season, is a Toyota-backed driver and set to leave after Williams decided not to continue with the manufacturer's engines.