The gun was discovered Wednesday night during security screening at Sea-Tac International Airport, KIRO-TV, Seattle reported. An airport spokesman said Russell had a permit for the .38-caliber Smith & Wesson.
Russell was briefly held at the airport and released.
Lori Dankers, a spokeswoman for the Transportation Security Administration, said Russell could be fined from $3,000 to $7,500 in civil penalties. Any charges would be up to state and local agencies, she said.
Guns, ammunition and replicas are banned from carry-on baggage. Passengers can transport unloaded weapons in checked baggage but must declare them at the airport.
Russell, 79, was a center with the Boston Celtics from 1956 to 1969. He later coached for the Celtics, the Seattle Supersonics and the Sacramento Kings.
He is considered one of the greatest players in the history of the NBA, which has named the NBA Finals MVP award after him. He was given a presidential Medal of Freedom in 2010 for his service to civil rights.