Known for his passionate play,[1][2] he was a six-time first-team All-Pro, twelve-time Pro Bowl selection, and named to the NFL 1990s All-Decade Team.

Originally from San Diego, California,[3][4] Seau played college football for the USC Trojans, earning All-American honors in 1989.

Later studies by the National Institutes of Health (NIH) concluded that Seau had chronic traumatic encephalopathy (CTE), a brain disease that has also been found in other deceased former NFL players.

[5][6] It is believed to be caused by repetitive head trauma, and can lead to conditions such as dementia, rage, and depression.

As a football player, Seau was a starter at linebacker and tight end, and as a senior, he was named the Avocado League offensive MVP and led the 18-member Oceanside Pirates team to the San Diego 2A championship.

[8] In basketball, as a senior, he was named the California Interscholastic Federation San Diego Section Player of the Year.

There's a lot of jealousy among Samoans, not wanting others to get ahead in life, and my parents got an earful at church: 'We told you he was never going to make it.'"

[8] Seau lettered in his final two seasons with the USC Trojans, 1988 and 1989, posting 19 sacks in 1989 en route to a unanimous first-team All-American selection.

[12] Seau quickly became one of the most popular players on the Chargers,[7] receiving the nickname "Tasmanian Devil", after the wild antics of the cartoon character.

[1][2] Seau started 15 of the 16 games he played in during his rookie season, and was named an alternate to the 1991 Pro Bowl after recording 85 tackles.

[19][20][21] He started no fewer than 13 games for the Chargers over each of the ensuing 11 seasons, registering a career high with 155 tackles in 1994, when he led his team to a championship appearance in Super Bowl XXIX.

He started five of the first seven games he played in with the Dolphins in 2005, but was placed on injured reserve on November 24 with an achilles tendon injury.

[29] On December 22, 2008, a fan was arrested for trespassing, assault, and battery for tackling Seau as he stood on the New England sideline during a home game against the Arizona Cardinals.

Seau announced his intention to retire permanently on the television program Inside the NFL on January 13, 2010.

Seau was actively involved with community work through Samoan "sister city" projects within San Diego County.

In 1992, Seau created the Junior Seau Foundation with the mission to educate and empower young people through the support of child abuse prevention, drug and alcohol awareness, recreational opportunities, anti-juvenile delinquency efforts and complementary educational programs.

[40] The 20th Anniversary Junior Seau Celebrity Golf Classic was held March 10–12, 2012, at the La Costa Resort and Spa.

[47] Seau sustained minor injuries in October 2010 when his SUV plunged down a 100-foot cliff in Carlsbad, California, only hours after he was arrested for domestic violence following an incident reported to the police by his girlfriend at their home in nearby Oceanside.

[53] He left no suicide note, but did leave a piece of paper in the kitchen of his home with lyrics he scribbled from his favorite country song, "Who I Ain't."

"[60] Seau had insomnia for at least the last seven years of his life, and he was taking zolpidem (Ambien), a prescription drug commonly prescribed for sleep disorders.

[61][62] Seau's autopsy report released later in August 2012 by the San Diego County medical examiner indicated that his body contained no illegal drugs or alcohol, but did show traces of zolpidem.

[70][72] On January 10, 2013, Seau's family released the NIH's findings that his brain showed definitive signs of CTE.

Russell Lonser of the NIH coordinated with three independent neuropathologists, giving them unidentified tissue from three brains including Seau's.

[76] Seau is one of at least 345 NFL players to be diagnosed after death with chronic traumatic encephalopathy (CTE), which is caused by repeated hits to the head.

[38] Bill Belichick, his coach at New England, praised Seau's leadership and willingness to accept any role.

He was inducted into the San Diego Chargers Hall of Fame on November 27, 2011, as part of Alumni Day ceremonies at a sold-out game against the Denver Broncos at Qualcomm Stadium.

On September 16, 2012, the Chargers retired Seau's number 55 during a ceremony at the 2012 regular season home opener against the Tennessee Titans.

However, the Hall of Fame cited a five-year policy of not allowing speeches for deceased inductees, denying Sydney the opportunity to introduce her father.

[87][88][89] Instead, she was allowed to speak onstage for three minutes uninterrupted on the NFL Network, and delivered a pared down version of her full speech, which The New York Times published.

[90][91] On August 5th, 2023, Junior Seau's linebacker teammate in Miami for three years, Zach Thomas, paid tribute to him at the end of his Pro Football Hall of Fame speech: "But there's one person whose name I've yet to mention.