San Diego Chargers Chaplain Shawn Mitchell told ABCNews.com that Seau died of a "self inflicted gunshot wound to the chest this morning." Seau was 43.
The Chargers released a statement to ABC News' San Diego affiliate: "Everyone at the Chargers is in complete shock and disbelief right now. We ask everyone to stop what they're doing and send their prayers to Junior and his family."
Seau was a standout linebacker with the University of Southern California before going to the San Diego Chargers, whom he led to the Super Bowl following the 1994 season.
He remained with the Chargers until 2003 and went on to play with the Miami Dolphins and New England Patriots before retiring after the 2009 season.
The Associated Press and ABC News Contributed to this report.