Taking to Instagram on Monday, the star posted two pictures, one showing them smiling to photographers as they made their way into Windsor's town hall on 21 December 2005, where they entered into a civil partnership, and a second, which was taken at their wedding reception in 2014, when they formally converted their civil partnership into a marriage.
WATCH: Elton, David and their two kids get festive ahead of Christmas
Captioning the snaps, David wrote: "I can't believe that 15 years has passed since @eltonjohn and I celebrated our Civil Partnership in Britain. What a wonderful day that was. 6 years ago, we renewed our vows and got legally married in front of our two beautiful sons. This type of civil progress was unimaginable when I was growing up.
"Yet despite all this extraordinary positive change, I remind myself that it's still illegal to be LGBT+ in 70 countries, and that you can be given the death penalty in 12 of those same countries.
"Through our work with the Elton John AIDS Foundation, Elton and I pledge to continue the fight for equality for our LGBT+ brothers and sisters around the world," he added.
David shared a picture from the 2014 wedding reception
Friends of the couple were quick to send them their best wishes, with Reggie Yates writing: "Love this. Here's to the next 15..."
Scottish actor Craig McGinlay added: "Love this David. Here's to many more happy years together," whilst a fan remarked: "Congratulations!! And Love is love!!"
David and Elton documented their big day back in 2014 on social media, and the stunning ceremony, which took place at their Windsor estate in Berkshire, was attended by the likes of David and Victoria Beckham, musician Ed Sheeran and actor David Walliams.
Back then, Sir Elton posted several snaps from his big day, including a picture taken at a window showing a garden, with the caption: "Good morning! Nice day for a wedding."
He later wrote: "The tables look stunning! We had red roses at our Civil Partnership 9 years ago and they brought us so much luck."