The 45-year-old, who was previously engaged to Sheryl Crow, announced that Anna Hansen had accepted his proposal by sharing a picture of the couple looking at each other and smiling while sat on the back of a boat on Lake Austin in Texas.
The couple have been together almost 10 years and have two children Max, seven, and Olivia, six, together but, although they've finally taken their relationship to the next level, it's not yet known whether they have an idea on a time and place for their wedding. He shares three children - son Luke, 17, and 15-year-old twin girls, Grace and Isabelle - with first wife Kristin Richard.
He said: "It was a good ride". She's been a pillar of strength and love for me and our entire family.
He won a record seven consecutive Tour de France titles during an illustrious cycling career, before having the titles stripped in 2012 for having used performance-enhancing drugs.
"Nine years ago today I met this awesome woman in Denver, Colorado", Armstrong wrote. "As y'all can imagine, the last half of those nine years hasn't exactly been smooth", he captioned a photo of the two of them together. "Happy Anniversary babe. I love you!"