Skipper Alan Dunne will end his 22-year association with Millwall this summer as one of 18 players released by manager Neil Harris.
The 32-year-old defender made 388 appearances in total for the Lions and scored 17 goals.
Chairman John Berylson told the club's official website: "It is always sad when a member of your family who has lived with you for a long time leaves home.
"Like Neil Harris and Paul Robinson before him, Alan leaves us with our thanks and very best wishes for the future. He has written his own chapter in Millwall's history and he will always be very welcome whenever he returns to The Den.
"While Alan will continue his playing career elsewhere, it would not surprise me to see him back here in some capacity further down the line."
Dunne is joined by fellow first-teamers Nicky Bailey, Matthew Briggs, Richard Chaplow, Ricardo Fuller, Carlos Edwards, Magaye Gueye, Justin Hoyte, Stefan Maierhofer, Angel Martinez, Danny Shittu and Martyn Woolford as casualties of Harris' clear-out following relegation.
Youngsters Dylan Casey, Sofiane El-Bekri, Denzel Gerrar, Jake Goodman, Josh Siafa and Callum Webb have also been released.
Harris has confirmed Jimmy Abdou and John Marquis will be discussing new contracts while Edwards has been invited to continue his rehabilitation from a long-term knee injury at the club.