After five years at Craven Cottage, Duff is to become a free agent at the end of his contract.
He leaves having made close to 200 appearances and with plenty of memories to look back on, including a Europa League final appearance and back-to-back top-half finishes in the Premier League.
Duff admits to having thoroughly enjoyed his time at Fulham, but accepts that it is now time for him to embark on a new challenge.
He told the club's official website: "I think it was the right time for everyone, for me and the club, to part ways. It's sad but that's the way it is.
"I've matured as a person and a player at Fulham. Since I've been here I've got married, I've had two kids, so it's been a special time in my life and I stand by what I've said before that my most enjoyable time in football has been at Fulham.
"I've had many a great night at the Cottage, whether it be European nights or Premier League games. It's a wonderful place to play - I don't think there's any ground around like it, so it's a special place."
Duff's final season ended on a low, with a knee injury ruling him out of action as Fulham suffered relegation out of the Premier League, but the former Republic of Ireland international leaves with no regrets.
He added: "All the bad emotions hit me in February when I kind of knew I wasn't going to play again for Fulham, so I had my little cry then.
"It was my last day on Thursday so it was obviously sad saying goodbye to the people who I now call my friends for life. But I knew back in February that I'd played my last game for Fulham so that was a sad time as well.
"It's been a big part of my life for five years - I've loved every minute of it at Fulham Football Club.
"It's a great club but, as sad as it is to say, we deserved to go down. The table doesn't lie, whether you win it or you go down, so I love the club but we are where we are."