His Royal Highness, who has repeatedly spoken out for the persecuted church, said he was praying for Copts in Egypt and around the world.
In the letter to Pope Tawadros II, he said: "The steadfastness of Egyptians in the face of such appalling hatred and extremism is greatly to be admired.
"I can only begin to imagine the dreadful shock and grief that Copts, and indeed all Egyptians must be feeling and my heart goes out to the families and loved ones of those who have been killed and wounded.
"I wanted to assure Your Holiness that Copts everywhere, and especially in Egypt, are in my thoughts and special prayers as they try to cope with the loss, pain and fear that the attack has inflicted upon them."
Twenty-four people died when a suicide bomber entered a chapel next to St Mark's Cathedral.
Police have arrested four people in connection with the attack.