The 6ft 9in power forward/centre is the fifth player to commit to the Raiders for the 2015/16 season, following in the footsteps of fellow Australians Daryl Corletto, Bennie Lewis and Josh Wilcher and New Zealander BJ Anthony.
MacMillan previously played with Corletto at Melbourne.
The 28-year-old went to Gardner Webb University in the United States and spent one season playing in Germany. However, apart from that all his career has been spent in the Australian NBL.
“To say I’m excited about this upcoming season with Plymouth is a massive understatement,” said MacMillan. It’s a fantastic opportunity and I flat out can’t wait.
“I think we are going to play an exciting brand of basketball this season and with the squad Jay (Marriott) has put together we should be legitimate championship contenders.”