Antetokounmpo spent the past two seasons with Panathinaikos in the brothers' native Greece. Another brother, Kostas Antetokounmpo, is a member of the Dallas Mavericks.
Thanasis averaged 5.4 points and 3.7 rebounds in 19 Greek Basket League games along with 4.2 points and 1.8 rebounds in 29 Euroleague appearances for Panathinaikos during the 2018-19 season.
The 26-year-old Athens native's only NBA experience came during the 2015-16 campaign when he played six minutes over two games with the New York Knicks.
His signing with the Bucks comes just a few days after the family opened the Antetokounbros Academy in Athens for youth basketball players.
"Our goal is helping others climb to the top," Thanasis wrote Thursday on Instagram. "Working with Nike Basketball, the Onassis Foundation and EuroHoops to start Antetokounbros Academy to teach basketball and life skills in Athens to help kids reach their dreams."
It's unlikely the newest Antetokounmpo on the Bucks plays a significant role since Giannis and Co. are a legitimate championship contender with one of the league's deepest rosters heading into next season.
Thanasis could end up splitting time between the NBA and the G League, providing forward depth during his stints in Milwaukee.