The guard's agent, Jeff Fried, said Wednesday the decision "wasn't about Dallas as much as it was about what's best for Monta."
Ellis was set to make $8.7 million in the final year of a three-year, $25 million contract. Fried didn't rule out a return to Dallas, but the Mavericks are expected to make a push for either All-Star forward LaMarcus Aldridge or center DeAndre Jordan in free agency.
The 29-year-old Ellis led Dallas at 18.9 points per game last season. He is one of three starters headed to free agency along with point guard Rajon Rondo, who won't return, and center Tyson Chandler.