"My dear friend of 29 years, Luke Perry, was one of those truly special people who really cared," Priestley wrote.
He added: "Luke was not only a star, he was an incredibly bright light that was extinguished far too soon... and that is why I, and so many others, are in so much pain today... If you had the good fortune to know Luke, or to have ever crossed his path, I know you are sad today as well... the candle that burns twice as bright only burns for half as long... you burned oh, so brightly Luke... Goodnight Sweet Prince."
The message accompanied photos of the actors posing happily together recently and in their 90210 heyday.
Perry died Monday after suffering a massive stroke last week. He was 52 and working on The CW drama Riverdale at the time of his death.