The singer wrote: "29 years ago today, I was a broken man.
"I finally summoned up the courage to say 3 words that would change my life: "I need help." Thank you to all the selfless people who have helped me on my journey through sobriety.
"I am eternally grateful."
Many fans have congratulated the star on his incredible milestone, with one commenting: "You are an inspiration to the world in so many ways. Thank you for sharing. You are an awesome person. Your 29 years of sobriety and your courage and strength set an example for others."
Someone else wrote: "Great work Elton! You are such an inspiration and a great great man. Very talented. So congratulations on your 29th year!!"
Another said: "It takes great courage to admit to yourself that you need to make changes, and even greater courage to say it out loud and then follow through on those necessary actions. Congratulations Elton!"
A fourth added: "We're all so proud of you, Rocketman."
"The life I was leading, flying on the Starship [his legendary private plane], living in beautiful houses, buying things left, right and center - it was not a normal life, not the sort of life I came from anyway," he said.
"I lost complete touch with that. I vowed when I did change my life that that would never happen again."
Rocketman documents the superstar's rise to fame, but doesn't shy away from showing some of the less glamorous details about Elton's life, including his struggles with drugs, bulimia and mental illness.
Now happily married to David Furnish, his partner of 26 years, John has two sons: Zachary Jackson Levon, eight, and Elijah Joseph Daniel, six. Last year, he also announced he would be retiring from touring so that he can spend more time with his family.
"I am a survivor," John added.
"I've survived a lot of things. Life is full of pitfalls, even when you're sober. I can deal with them now because I don't have to run away and hide."