The 49-year-old British actress paid tribute to Ledger in an Instagram post Tuesday on what would have been the late actor's 39th birthday.
Watts, who dated Ledger from 2002 to 2004, posted a throwback photo of herself and the star attending the 2004 Screen Actors Guild Awards.
"Happy birthday to this darling heart. We will never forget you... #heathledger #rip," she captioned the post.
Watts also shared a photo with Ledger on Instagram Stories, writing, "Beautiful Heath."
Ledger died at age 28 in January 2008 following an accidental drug overdose. Watts, who co-starred with Ledger in the 2003 movie Ned Kelly, dedicated a post to the actor on the 10th anniversary of his death in January.
"Thinking about this beautiful soul today. 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. I will never forget his gentle spirit," she wrote.