Middleton made his NBA debut for the Detroit Pistons before moving to the Bucks in 2013 and has since cemented himself as one of basketball's top sharpshooters. A playmaker for the Bucks, Middleton was named an NBA All-Star in 2019 and 2020.
Bullets majority owner Martin said he wasn't the NBL hate attracted Middleton and that he was looking forward to working alongside the small forward.
"Khris has thousands of opportunities to invest money into different sectors so we are excited to have him as part of our team," said Martin.
"Though he has ten more years in the NBA to play, with this opportunity he will be able to be a part of a professional basketball ownership journey in one of the most coveted places in the world.
"Someone of Khris's stature investing in the Brisbane Bullets organisation represents the ongoing, unprecedented rise of the NBL. Khris and the rest of the sports world are quickly realising the quality of the league."
The Bullets also welcomed Kyle Hines and Melissa Oden to the ownership group.
Hines, the 2007 Southern Conference Player of the Year and a four-time EuroLeague champion, is currently playing for Olimpia Milano in Italy, while Hines becomes the first African-American woman to purchase a stake in an NBL team.