The deal comes after the Sacramento Kings rescinded their qualifying offer to Cauley-Stein, making him an unrestricted free agent.
The 25-year-old's agent, Roger Montgomery, recently said his client was hoping for a new home and more opportunities.
"I really think Willie needs a fresh start," said Montgomery. "Based on how things have gone for him there in Sacramento, I just think it's time for Willie to move on and we'd really like him to move on."
Cauley-Stein is expected to earn slightly more than the league minimum, sources told ESPN's Zach Lowe.
The 7-footer is an active defender that can switch on all five positions while giving the Warriors a legitimate rim-running threat. He'll get a chance to flourish and become Golden State's starting center after being stuck in the Kings' loaded frontcourt.
Cauley-Stein averaged 11.9 points and 8.4 rebounds in 81 appearances during the 2018-19 season.