Fellow actress Kate Bosworth was one of the first to post a heartfelt message, remembering her friend as a "sensitive, gentle, kind, wickedly funny..extraordinarily special" person.
"Doesn’t seem so long ago we were all running around LA, shotgunned into Hollywood + popping off the new millennium’s cork, having a laugh. It was a crazy time. I was literally straight out of high school, feeling young & super shy and he was always so sweet within a surreal scene.
"I really, really would have loved to have known Heath today, at 40. No limits on what he would have accomplished. Here is what I remember: he was sensitive, gentle, kind, wickedly funny. Extraordinarily special. Time goes by, his image still a teenage heartthrob. And his work. So brilliant + honest & he was just getting started.
"But to remember the Human. I was on the outer orbit. Imagine those close to the sun. Thinking of all the people who knew and loved him on this day. Man, you made one hell of a mark. xo"
Entertainment Tonight, BBC Radio and E! News all remembered the icon with messages of love on Twitter.
Fans flooded the internet with posts about the Dark Knight legend, with one writing: "Heath Ledger would have been 40 years old today. The world was undeserving of such a beautiful soul. Rest in paradise, handsome."
Another referenced the release of the promo for the upcoming Joker film - Ledger's defining role - tweeting: "Fitting that the new Joker trailer was released a day before Heath Ledger's birthday. I want to remind everyone how amazing he was as a human being and actor. Happy birthday."
The Alternative Press tweeted: "Today Heath Ledger would be celebrating his 40th birthday. Ledger was a cultural legend, taking each character to the next level and dedicating his entire being to his craft. Your work lives on as some of the best and we wish we’d had the chance to see more. Rest easy, Heath."