Police in Oceanside, California were called to the sportsman's home following reports of a shooting incident inside the property, where his girlfriend had found Seau unconscious in bed with a gunshot wound to the chest.
A firearm was found nearby and cops are investigating the 43 year old's passing as a suicide.
Tributes have poured in from friends, colleagues and fans of the football icon, a University of Southern California (USC) star who helped to take his former team the San Diego Chargers to the 1995 Super Bowl. And a number of his celebrity pals have also remembered Seau via Twitter.com.
Peete, whose footballer husband Rodney played at USC with Seau, writes, "Paralyzed with grief over the loss of our dear friend Junior Seau... Husband devastated. I just don't want to believe it. Loved him...."
Lachey tweets, "So sad to hear about junior seau's passing. I loved him as a Trojan (USC player) and got to know him a little over the years. Great guy. rip", while Union, who met Seau while filming in San Diego in 2000, adds: "Deepest sympathies go out 2 the family, friends & fans of JuniorSeau ...great guy who made the 'Bring it On' cast at home at his restaurant 'Seaus' in SD".
Gladiator star Russell Crowe, Grey's Anatomy hunk Jesse Williams, and Tom Cruise's DJ son Connor have also posted tributes online.