A young woman who identified herself as Seau's girlfriend alerted police to the death, saying she had found him wounded and unconscious in his beachfront home just north of San Diego, Oceanside police chief Frank McCoy told reporters.
The death was at least the third apparent suicide by a former NFL player since February 2011, when former Chicago Bears defensive back Dave Duerson killed himself in a case that led to increased scrutiny of the effects of head injuries in football.
Seau, a 12-time Pro Bowl selection whose last NFL season was in 2009, played most of his 20-season career with the Chargers before moving to the Miami Dolphins and the New England Patriots. He was named to the Pro Football Hall of Fame's All-Decade Team of the 1990s.
Seau's mother, Luisa, was weeping as she spoke before television cameras outside her son's house, where family and friends gathered as police continued to work at the scene.
"I pray to God please take me, leave my son," she said. "But it's too late."
Seau was inducted last year into the San Diego Chargers Hall of Fame, and was expected to be considered for the NFL Hall of Fame. He will become eligible for induction in 2015.
His death was being investigated as a suicide, police said.
Seau, however, did not have a documented history of concussions.
He suffered a broken arm in 2006 that caused him to miss games, and before that sat out some contests with injuries to his chest muscle and Achilles tendon.
Seau was found in the bedroom at his home, and investigators recovered no suicide note, Oceanside police spokesman Lieutenant Leonard Mata said. The weapon Seau apparently used on himself was a revolver, Mata said.
In 2010, hours after his arrest on suspicion of domestic violence, Seau drove his car off a cliff in what he said at the time was an accident. Seau said he fell asleep at the wheel, and prosecutors later declined to file charges in the domestic violence complaint.
His death came less than a month after former Atlanta Falcons safety Ray Easterling shot himself to death at age 62.
Easterling had been diagnosed with dementia.
Hundreds of former NFL players and their families have joined in legal action against the NFL, claiming negligence for not doing enough to lessen the risks from head injuries.
Duerson's family filed a wrongful death suit against the NFL, arguing that a series of concussions he suffered during his NFL playing career damaged his brain. Duerson, who also shot himself in the chest, had told friends he wanted his brain examined after his death.
The league, in the face of criticism, has focused in recent seasons on health and safety issues. It has cracked down on hits to the head, and stiffened rules that prohibit players using their helmets as a weapon by making contact head-first, which is subject to fines and suspension for repeat offenders.
At Seau's home, across the street from the Pacific Ocean, his mother sat on driveway next to a coroner's van and sang a song or hymn. Over 30 family members and friends joined in before the van carrying Seau's body left the property.
Luisa Seau followed the van down the driveway with her arms outstretched, crying and wailing, and was held back by several people. On the dashboard of the coroner's vehicle was a Chargers placard.
Seau was born in San Diego and showed a love for surfing. In the garage of his two-story, brick and stucco Oceanside home were several surfboards neatly racked against a back wall, near a silver Mercedes-Benz.
"Everyone at the San Diego Chargers is in complete shock and disbelief right now," the team said in a statement released on its Facebook page. "We ask everyone to stop what they're doing and send their prayers to Junior and his family."
Fans left bouquets of flowers and cards on a low brick wall outside the house, and the NFL expressed condolences.
"It is incredibly tragic and sad," NFL spokesman Greg Aiello said in a statement. "Our prayers are with Junior's family."
Seau was a fixture in San Diego's social and philanthropic circles. He opened a restaurant in town, and had a charity called the Junior Seau Foundation that was dedicated to helping young people avoid child abuse, drugs and alcohol.
Fagatua Tili, a pastor and the chaplain for Oceanside police, knew Seau from when the athlete was a school boy. Like Seau, Tili is of Samoan descent.
"Junior was a good man. He was a good boy and very kind. The talented gift he had he used well," Tili said. "I wish he asked for help. I wish we knew he was troubled."