Sauber said in a statement on Monday that, as planned, the Volkswagen-owned brand took the stake this month but gave no financial details.
"This is an important milestone on the way to Audi’s entry in Formula One," it said.
Audi announced last October that it had agreed to take a stake in Sauber Group, which has been involved in Formula One since 1993 and whose team are competing this season as Alfa Romeo using Ferrari engines.
Audi will make its own F1 power unit in Neuburg, Bavaria.
Reuters