In a message posted to his 317,000 followers, the Oscar nominee thanked Charles for visiting his wife Joan Washington prior to her passing last year after a battle with lung cancer.
Grant wrote: "Will never forget the kindness of the King, who visited my wife in her final weeks — empathetic, philosophical, compassionate and kind.
"My heart goes out to him for the incalculable loss of his beloved mother."
Grant, who has dual citizenship of the UK and Commonwealth country Eswatini, shared pictures of the occasion on which he met the King last year as part of the Prince's Trust Awards.
The 65-year-old actor was also able to meet the Queen as part of his connection to the Royal Academy of Arts.
Grant met his wife Joan — a voice coach — via work and the couple married in 1986, later having a daughter together.
In a touching tribute to his wife penned for the Daily Mail, Grant wrote that their relationship was "the ride of my lifetime".
He added: "It's an extraordinary phenomenon to be truly 'seen' and 'known' by another human, and in Joan, I found someone who innately did both."
Grant is best known for his breakout role in the British comedy Withnail & I, as well as the 2018 movie Can You Ever Forgive Me? — for which he received an Oscar nomination.
The star's tribute message was one of many shared by famous faces ahead of the Queen's funeral, with Victoria Beckham among those sharing their reflections on the death of the monarch.
She said in an Instagram post that the Queen was the “ultimate icon in every way and the epitome of elegance”.
Watch: Richard E. Grant reveals memoir title inspired by his wife