Wanamaker's agent, Steve McCaskill, told the news to ESPN's Adrian Wojnarowski, who noted that the ex-Boston Celtic is "expected to see some significant minutes" backing up two-time NBA MVP Stephen Curry.
The 31-year-old averaged 6.9 points, 2.5 assists and 2.0 rebounds for the Celtics in 71 games last year. He averaged 19.3 minutes per game, primarily as a reserve.
Wanamaker went to Pitt from 2007-2011 and was named a honorable mention NCAA Men's Basketball All-American during his senior year.
The guard went undrafted and proceeded to play professionally overseas in Italy, France, Germany and Turkey from 2011-2018.
He finally landed in the NBA after signing a one-year deal with the Boston Celtics in 2018. Wanamaker shot 47.6 percent from the field and 41.0 percent from three-point range during his rookie year while posting 3.9 points in 9.5 minutes per game.
The C's re-signed Wanamaker for the 2019-20 season, but he's now headed to the Bay Area for a year with the Warriors.
Reviews of the signing were mostly positive from analysts on Twitter.
"Really happy for Brad Wanamaker!" Keith Smith of Yahoo Sports and Celtics Blog wrote.
"He was way better for Boston than he ever got credit for. Warriors got a good one."
Monte Poole of NBC Sports Bay Area also provided his take: "Brad Wanamaker is a solid backup PG who also can slide to SG. Smart. Plays both ends. Warriors don’t need excitement there. Very good value at $2.25 mil. Still have MLE ($5.7 mil) available for a veteran big."
Wanamaker joins a Warriors team that figures to start Stephen Curry, Andrew Wiggins, Kelly Oubre, Jr., Draymond Green and James Wiseman. Klay Thompson, the team's usual shooting guard, is expected to miss the 2020-21 campaign with a torn Achilles.