The One Tree Hill actor, 33, shared the moving message to his Instagram page on Tuesday, 18 August, alongside a collage of photographs of himself and his beautiful companion.
He wrote: "My best bud & 1st born son Joe, thank you for everything you've ever done for me. You've been my kindred spirit, my unconditional friend. Thank you for carrying the weight that I could not. You taught me more about selflessness and compassion than I thought a dog could know."
Chad continued: "Thank you for always keeping the status Quo. Even the dogs at the dog park thank you for keeping the peace. Almost 15 years of adventures Joe and I'm so grateful for every minute. Rest easy now bud. Check in on us once and while would ya... You're missed and always will be. Thx for waiting to meet my son. This family won't be the same without you but so much stronger because of you. Rest in peace Joey. We Love You, Chad, Sarah & Baby Murray."
Thousands of fans left comments revealing how touched they were by the letter. Many also shared stories of their own pets and dealing with their losses.
One fan (@amazedbyjasv) wrote, "This brought me to tears!! Amazing how much you learn about life with dogs. Just beautiful," while another (@cheyennemc23) said: "I am just literally crying right now. That was so tear jerking... I lost my dog that I have had for 14 years and it was hard I lost another and it is just heartbreaking. I miss them every day and always will miss them."