In a deeply personal statement, signed "Charles R", he said he has been uplifted by their encouragement after his diagnosis was revealed on Monday.
King Charles said: "I would like to express my most heartfelt thanks for the many messages of support and good wishes I have received in recent days.
“As all those who have been affected by cancer will know, such kind thoughts are the greatest comfort and encouragement.
"It is equally heartening to hear how sharing my own diagnosis has helped promote public understanding and shine a light on the work of all those organisations which support cancer patients and their families across the UK and wider world.
"My lifelong admiration for their tireless care and dedication is all the greater as a result of my own personal experience.”
It comes after both Queen Camilla and The Princess of Wales thanked people for their kindness while carrying out public engagements this week.
When asked about Charles's health as she attended a musical event at Salisbury Cathedral, Camilla said: "He is very touched by all of the letters and messages the public have been sending from everywhere. That's very cheering."