According to ProFootballTalk.com, the NFLPA opened an investigation after learning of a fight during practice between cornerback Richard Sherman and wide receiver Bryan Walters, the special teams player who was promoted from the practice squad last December to fill suspended cornerback Brandon Browner's roster spot.
TV reports reached the NFLPA, which requested practice video, which reportedly showed excessive contact throughout the practice period.
Head coach Pete Carroll was fined $100,000 and the team $200,000.
The NFL confirmed that the Seahawks were accused of allowing players to engage in "excessive levels of on-field contact" during the team's mandatory minicamp for veterans on June 16.
In addition to the financial penalty, the team will lose two days of on-field two-a-days for veterans and will have its final day of minicamp limited.
Players will be paid for the cancelled practice sessions.
Carroll and the team also were fined in 2012 for a similar violation.