He said: “I am very proud of my son, William, for his growing commitment to the environment and the bold ambition of the Earthshot Prize.
“As a world, we need to come together to inspire, reimagine and build the sustainable future we so desperately need.”
He signed the tweet: “HRH The Prince of Wales”.
Celebrities such as Emma Thompson, Emma Watson and David Oyelowo were also in attendance.
No stars have flown to London for the event at Alexandra Palace.