Watts, 49, took to Instagram to share a black-and-white photo of Ledger, who passed away in 2008 from an accidental overdose in his Manhattan home.
"Thinking about this beautiful soul today," she began in a caption. "10 years ago he left this world. He was a true original."
"Heart always on his sleeve, with the most powerful charisma, strength, humor and talent," Watts concluded. "I will never forget his gentle spirit."
Watts and Ledger began dating in 2002. The two fell in love on the set of the 2003 film, "Ned Kelly." Their relationship ended in 2004.
The actress opened up about her relationship with the "Brokeback Mountain" actor, telling More magazine, "We had a beautiful relationship, only a couple of years, but he was a man who was completely full of joy, and there was a lot of laughing and affection."
"He was really a very special soul and made a great impact on my life. And a great actor, but I know there was so much more to come," she continued in the 2011 interview.