William Shatner is saying goodbye to his fellow ‘Star Trek’ captain Patrick Stewart … remembering the late actor as a dear friend and a true legend of stage and screen.

William paid tribute to Patrick in a social media post Thursday, writing … “What a heartbreaking week it has been. Sir Patrick Stewart was a friend, a colleague, and a magnificent actor.”

He continued … “From the stage to the bridge of the Enterprise, he brought intelligence, grace, and a deep sense of humanity to everything he did.”

William, who famously played Captain James T. Kirk in the original “Star Trek” series, shared the screen with Patrick’s Captain Jean-Luc Picard in the 1994 film “Star Trek: Generations” … which brought the two iconic commanders together.

Their friendship continued long after that film … with Patrick appearing in William’s 2011 documentary “The Captains,” in which the actor interviewed fellow “Star Trek” stars about their experiences leading the franchise. Patrick also appeared in his 2015 documentary “Chaos on the Bridge.”

In his emotional tribute, William said Captain Picard “stood for the best of us, and so did the man who played him” … adding he was filled with affection and admiration for Patrick and his work.

William ended by sending love to Patrick’s wife, Sunny, and his family … saying the actor would be deeply missed.

As TMZ previously reported … Patrick died Wednesday at 86 following a battle with Alzheimer’s disease and Lewy body dementia.