The American, 37, who joined Everton on loan from Manchester United in 2006 before signing permanently in 2007, will join Colorado Rapids this summer.
"After a decade in blue, this is my home," Howard wrote on social media.
"I love all of you, and am so grateful for what the club and everybody affiliated with it has given me."
Howard, who has 107 caps for the United States, won the 2004 FA Cup and 2006 League Cup with Manchester United, but has failed to win a trophy with the Toffees during his time working under managers David Moyes and Roberto Martinez.
"I remember the promise that David Moyes made to me 10 years ago: 'once you walk through those doors, you'll fall in love'.
"Today, I will walk through those doors one more time, and then I will depart.
"I will depart with love in my heart for the greatest of football clubs, the people's club, Everton FC.
"I will always be thankful to so many. Of course, the fans - those who bring such passion and make Everton what it is. My team-mates. Among others, Tony Hibbert, Leon Osman and Leighton Baines were always a shining example of how a true Evertonian should train, play and act.
"Phil Jagielka has been the greatest and strongest captain, and it has been a privilege to follow him. Two great managers. David Moyes was the ultimate leader, combining strength and integrity. I am greatly indebted to Roberto Martinez for his decency and commitment to me as a player and a man.
"My wonderful chairman, Bill Kenwright, whose approving smile from the directors' box will be an enduring memory; words cannot express my gratitude.
"And there are so many others that work at the club or support the club who are all part of my family, now and forever.
"I will soon walk out on the Goodison pitch, the 414th time I have been honoured and trusted to wear the famous Everton shirt. I will walk out of the tunnel one more time to hear Z-Cars and the thunderous applause of 40,000 Evertonians. I will hold these images and sounds with me wherever I go.
"I am going home to America. But after a decade in blue, this is my home. I love all of you, and am so grateful for what the club and everybody affiliated with it has given me.
"Everton, you have become a part of my soul. You'll forever be in my heart. COYB!"
Everton keeper Tim Howard has penned an emotional farewell letter ahead of his 414th and final appearance, stating the club has "become part of my soul" .