Robert Irwin has shared a photo of a note he was given by a stranger on a plane.

As Robert, 15, and his family caught a flight on Father's Day in the US (June 16), a fellow passenger decided to scribble a message on a napkin and have it passed onto the Irwins.

"Our lovely flight attendant just handed us this note on our way off the plane," Robert wrote on Instagram where he shared a photo of the note.

"Little gestures like this really mean a lot. It warms my heart to see people who take a little bit of time to do something kind. Thank you Carolyn."

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The note said: "I want to acknowledge your beautiful family on Father's Day! I'm sure he is proud of all of you!"

Robert earlier paid tribute to his father Steve, who was killed in 2006, by posting a sweet photo of himself and Bindi with their dad.

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You’ll always be the best dad in the world ❤️

A post shared by Robert Irwin (@robertirwinphotography) on

He captioned the photo: "You'll always be the best dad in the world."

Bindi also shared a throwback photo to celebrate Father's Day, writing, "Thank you for always teaching me to stand up for what I believe in. I'm incredibly blessed to be your daughter and I love you so much."