The team says the 87-year-old Modell is at Johns Hopkins Hospital.
Kevin Byrne, senior vice president of public and community relations for the team, did not know Modell's condition.
Modell purchased the Cleveland Browns for $4 million in 1961 and moved the franchise to Baltimore in 1996. The relocation made him a hero in Baltimore but a pariah in Cleveland.
He sold a minority interest in the team to Steve Bisciotti in 2000 (a season in which the Ravens won the Super Bowl), and in April 2004, Bisciotti completed purchase of the franchise. But Modell retained a 1 percent piece of the team and is listed in the current Ravens media guide as a minority owner.
During his days as sole owner of the team, Modell served as NFL president from 1967-69, chaired negotiations for the first collective bargaining agreement with the players in 1968 and was a key figure in getting NFL games televised on Monday nights.
Modell enjoyed watching the Ravens practice, always riding in a golf cart because he had difficulty walking. But those visits became more infrequent in recent years because of his failing health.
His wife of 42 years, Patricia, died in 2011.
The Associated Press contributed to this story.