Russell, 79, had a permit to carry the handgun and was released after being cited an undisclosed amount Friday.
"Before boarding my flight from Seattle to Boston, I had accidentally left a legal firearm in my bag," Russell said in a statement Saturday. "I apologize and truly regret the mistake. 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's .38-caliber Smith & Wesson was found in his carry-on baggage at a security checkpoint. TSA fines typically range from $3,000 to $7,500 for such offenses.
Russell, who won 11 NBA titles with the Boston Celtics, lives on Mercer Island, Wash., near Seattle.
He will be honored on Nov. 1 with a statue of his likeness in Boston's City Hall Plaza.