This isn't the first time 49-year-old Watts has shared a heartfelt message about Ledger. In January, she Instagrammed a picture of the late actor on the 10th anniversary of his death.
"Thinking about this beautiful soul today," she wrote at the time. "Ten 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. I will never forget his gentle spirit."
Watts and Ledger dated from 2002 to 2004. In September 2011, Watts opened up about her relationship with the fellow Australian actor, who died at age 28 from an accidental overdose of prescription medications.
"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," she told More magazine. "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. And it’s a tragedy for his little daughter."
Watts was referring to Ledger's daughter, Matilda -- who was only two years old at the time of his death -- with actress Michelle Williams. The notoriously private Williams opened up about raising Matilda without her father in November 2016.
"In all honesty, for pretty much everything else, I feel like I'm a believer in not fighting circumstances, accepting where you are and where you've been," she told Porter magazine. "In pretty much all senses but one. I would be able to go totally down that line of thinking were it not for Matilda not having her dad. You know, that's just something that doesn't… I mean, it just won't ever be right."