The incident happened Sunday at the end of the Oakland Raiders-Tennessee Titans game, CNN reported.
As onlookers yelled to the woman, "Don't do it, don't do it," the 61-year-old Marine vet, who also was not identified, softened her 40- to 50-foot fall with his body, authorities said.
"He put himself in a lot of risk. And, right now, he is in the hospital paying the price for it," Alameda County Sheriff's Sgt. J.D. Nelson said.
The woman suffered a serious head wound and was in a hospital intensive care unit early Monday, Nelson said. The man suffered non-life-threatening injuries.
"There are not a lot of people who would have done what he did," Nelson said.
It was unclear what prompted the woman to jump, Nelson said.
In an unrelated incident, a 48-year-old man suffered severe head injuries when he slipped and fell down steps at Baltimore's M&T Bank Stadium, CNN said.
The incident happened during the Ravens' game against the New York Jets, authorities said.