The couple reportedly started dating late last year.
"I can confirm that Amanda broke up with Doug," Melissa Raubvogel, the 22-year-old actress' representative, told People magazine exclusively.
Bynes is known for her roles in the films "Hairspray" and "What a Girl Wants," and for her work on the TV series "What I Like About You" and "The Amanda Show."
"She ended it. She had had enough," a source close to the actress told Usmagazine.com. "Her friends didn't trust him. Everyone thought he was using her for celebrity. He was not good, and she was over it. It was definitely her who ended it."
However, a source close to Reinhardt, told People the player isn't heartbroken about the split.
"Doug is so over being with her," the insider said.
Reinhardt, who played for the Baltimore Orioles before he was sidelined by a knee injury, also appeared on the television series "The Hills."
"She wouldn't even let me have a guys' night out without flipping out on me," Reinhardt told E! News exclusively. "So it's over."