The former Hills stars got together in 2007 and tied the knot two years later. They now have a seven week old son called Gunner together.
"Happy 9 year anniversary! It was love at first sight. I knew my life would never be the same. You are my soulmate, my heart, my soul, my best friend, my everything," Heidi captioned a photo of herself with husband Spencer and son Gunner on Instagram.
"We have fought through so much to be together. Every moment of it has been such a blessing. I knew I had to follow my heart and have faith in love and in you. Best decision of my life. Thank you so much for our son! This is been the best 11 years of my life.
"It continues to get better every moment, every day, every year. I love you more than I’ll ever be able to tell you! I didn’t think our love could get deeper until @gunnerpratt arrived! Not only are you the most amazing husband, you are the most amazing father! I can’t wait to see the journey life has in store for us!"
Heidi, 31, and Spencer, 34 - known together as 'Speidi' - appeared in Celebrity Big Brother last year, but were booted out one week from the finish line.
Despite reports of relationship break ups and marriage separations through the years the couple have remained a unit, appearing as a double-act for press calls and book signings both in the US and UK and now have a baby together.
Fellow Hills stars Whitney Port and Lauren Conrad have both also become mothers in the last year, with 31-year-old Lauren and her lawyer husband William Tell welcoming son Liam Jameson in July, and Whitney, also 31, welcoming Sonny Sanford Rosenman in August.