Former Boston Celtics center and head coach Bill Russell has apologised for an incident in which he was arrested for being in the possession of a gun at Seattle-Tacoma International Airport.

TSA agents found a loaded handgun, which the 79-year-old legally owned, inside Russell's carry-on bag before he was due to fly to Boston.

"Before boarding my flight from Seattle to Boston, I had accidentally left a legal firearm in my bag. I apologize and truly regret the mistake," Russell said in a statement.

"I was issued a citation by the TSA, whose agents couldn't have been more thorough and professional when dealing with this. I really appreciate their efforts to keep air travel safe."

Russell was named MVP of the NBA five times as a player.