Steve Morison will remain at The Den until the summer of 2019 after triggering an appearance-based clause in his contract.
In more than 250 appearances for The Lions, the Millwall forward has scored 86 goals, including the winner in the League One Play-Off Final that saw the club promoted to England's second tier.
The 34 year-old was delighted with the extension, telling millwallfc.co.uk:
"Some of the best football of my career I have played has been here at Millwall and the last couple of years have been unbelieveable.
'Still having moments like we did in May, winning the Play-Off Final, is great and to have them at my age is thoroughly enjoyable.
"I think we are doing alright this year and are ahead of where a lot of people thought we would be at this stage of the season. I'm really looking forward to the second half of the season."
Manager Neil Harris praised the veterans 'invaluable' input as a leader in the dressing room.
"His experience and leadership is pivotal to this club," the Lions boss said.
"His performances this year have been very, very good. He has picked up where he had last year and although he hasn't had the goal tally that we would like, his role is a little bit different this season.
"Where last year he was a talisman for goals, now he is our talisman for our attacking shape and a focal point.
"His knowledge of the game and what he can pass on to the younger players is really key.
"We are trying to build a young, vibrant squad and recruiting upcoming players and thinking long-term, but you have to have the balance. You have to have players like Steve Morison, Tony Craig and Dave Martin's because the players need guiding."