The lawsuit, filed Tuesday in Jackson County, Mo., Circuit Court by Cheryl Shepard, alleges Belcher, a linebacker, "unknowingly sacrificed his brain" in his four-year career with the Chiefs, and says the team failed to protect Belcher's safety, the Kansas City Star reported Wednesday.
It seeks unspecified damages.
Belcher, 25, shot his girlfriend, Kasandra Perkins, to death Dec. 1, 2012, at the home they shared. He then drove to the Chiefs' practice facility and killed himself with a single shot.
Belcher's body was exhumed, at the family's request, two weeks ago. His brain is being examined for evidence of chronic traumatic encephalopathy, a disease mentioned frequently in the lawsuit, although Belcher had no documented history of concussions or other head injuries, the newspaper said.
The lawsuit alleges the Chiefs missed signs of concussions, including one allegedly suffered in the last game of the 2012 season.