Hulkenberg's current team Haas announced on Friday morning that the German is leaving the squad at the end of the 2024 season.
The 36-year-old was out of contract at Haas and was long known to be on Audi F1 CEO Andreas Seidl's shortlist.
Hulkenberg will now join the team, which will still be known as Sauber in 2025, one year ahead of Audi's official F1 entry in 2026 on a multi-year deal. He previously raced for the Swiss-based team in 2013.
“I’m returning to the team I worked with back in 2013 and have fond memories of the strong team spirit in Switzerland,” said Hulkenberg.
“The prospect of competing for Audi is something very special. When a German manufacturer enters Formula 1 with such determination, it is a unique opportunity.
"To represent the factory team of such a car brand with a power unit made in Germany is a great honour for me.”
Hulkenberg and Seidl previously worked together at Porsche's WEC team, with the pair winning the 2015 Le Mans 24 Hours together on Hulkenberg's cameo for the Weissach squad, which Seidl was the team principal of.
Source: Autosport