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The findings were consistent with chronic traumatic encephalopathy, a degenerative brain disease widely connected to athletes who have absorbed frequent blows to the head, the N.I.H. said in a statement. Seau is the latest and most prominent player to be associated with the disease, which has bedeviled the sport in recent years as a proliferation of studies have exposed the possible long-term cognitive impact of head injuries sustained on the field.

"The type of findings seen in Mr. Seau's brain have been recently reported in autopsies of individuals with exposure to repetitive head injury," the N.I.H. said, "including professional and amateur athletes who played contact sports, individuals with multiple concussions, and veterans exposed to blast injury and other trauma."

Since C.T.E. was diagnosed in the brain of the former Eagles defensive back Andre Waters after his suicide in 2006, the disease has been found in nearly every former player whose brain was examined posthumously. (C.T.E. can only be diagnosed posthumously.)

Researchers at Boston University, who pioneered the study of C.T.E., have found it in 18 of the 19 brains of former N.F.L. players they have examined.

In July the N.I.H. began its examination of Seau's brain tissue and invited several neuropathologists to consult in the analysis. They found "abnormal, small clusters called neurofibrillary tangles of a protein known as tau within multiple regions of Mr. Seau's brain," according to the statement. Tau has been found in the brains of those with Alzheimer's disease and other progressive neurological disorders.

"I think it's important for everyone to know that Junior did indeed suffer from C.T.E.," Seau's ex-wife, Gina, said in an interview with ABC News and ESPN. "It's important that we take steps to help these players. We certainly don't want to see anything like this happen again to any of our athletes."

Shortly after Seau's death of a self-inflicted gunshot wound to the chest at his Oceanside, Calif., home in May, members of his family announced that they would donate his brain to the N.I.H. to be studied. The release of the results Thursday was at the request of the family, the N.I.H. said.

Seau's suicide was one of several high-profile suicides of former N.F.L. players, raising concern about head injuries in the sport. In February 2011, Dave Duerson, a former Chicago Bears player, shot himself in the chest, saying in a note that he wanted his brain donated for research. Doctors determined that Duerson had C.T.E. Duerson's son, Tregg, is suing the N.F.L., claiming the league did not meaningfully warn players about the potential risks of concussions.

Ray Easterling, a safety for the Atlanta Falcons in the 1970s and a plaintiff in a high-profile lawsuit against the league over its handling of concussion-related injuries, died just two weeks before Seau of a self-inflicted gunshot wound. Thousands of players and their spouses have joined the lawsuit, as well.

"The finding underscores the recognized need for additional research to accelerate a fuller understanding of C.T.E.," the N.F.L. said in a statement Thursday. Pointing to a $30 million research grant it has committed to the N.I.H. and $100 million it plans to invest in medical research, it added, "We have work to do, and we're doing it."

Over his 20-year N.F.L. career, Seau played for three teams, most prominently the San Diego Chargers, and made 12 Pro Bowls. He played in two Super Bowls and was named to the 1990s All-Decade Team by the Pro Football Hall of Fame.

After he retired from football after the 2009 season, Seau was known for his work with his foundation and his restaurant, often a cheery presence in and around San Diego. But friends said they struggled to understand some of Seau's darker moments. In October 2010, he drove his sport utility vehicle off a beach side cliff in Carlsbad, Calif., where it landed some 100 feet below the roadside. Seau claimed he fell asleep, but the timing seemed odd. That morning he was arrested on suspicion of domestic violence.