Cary Elwes is missing Rob Reiner a little extra today.

"The Princess Bride" star honored the late Hollywood icon with a heartfelt social media post Friday ... on what should have been Rob's 79th birthday.

Of course, Rob and his wife, Michele were stabbed to death in their home back in December ... and their son, Nick Reiner, is charged with their murders.

Cary posted a throwback photo of him and Rob, who directed the epic classic 'Princess Bride' back in the day.

The actor captioned the Instagram post, "Today would have been your 79th birthday. Still hard to believe you’re gone. Missing you so much #robreiner ⚔️💔."

It's Rob's first birthday since his murder ... so it's clear folks who adored him are hurting big time today.