Petrobras was a partner of the team between 1998 and 2009. Petrobras and Williams will work together to develop a new specialist motorsport fuel for use by the team starting in the 2015 season.
"Petrobras and Williams have had a successful partnership before and we are both looking forward to reuniting for 2014," said team principal Sir Frank Williams.
"Technologically they are very strong and that will be important for the team as the new regulations have made fuel efficiency increasingly important."
"They are also a very ambitious global company and we are looking forward to working together to further their marketing goals."
Williams has recently announced a continuation of their long-running partnership with Oris, as well as a new agreement with Genworth.
Williams is also expected to bring the Martini name and colours back into Formula 1 ahead of the opening round of the season in Australia. The team will continue to use their blue heritage livery at this week's test in Bahrain.
Brazilian GP2 driver Felipe Nasr is also expected to join the team as their reserve driver.