"Today i said goodbye to Sniper. he was my boy, my buddy and my best friend," the former The Bachelor star, 28, captioned a slideshow of sweet photos of his black Labrador retriever.
"I want to thank you for always being there for me. for getting me through some of the darkest times in my life. for always making me smile no matter what was going on in life. for finishing my left overs. for taking the blame of so many farts when [Cassie Randolph] and i started dating."
Colton also thanked Sniper for "always looking" at him "with love" in his eyes.
"I love and miss you already buddy. i promise to be the man you think i am. everyday," Colton added.
Colton also posted a photo on Instagram Stories of himself snuggling with Sniper on a couch.
"Love you pal!" Colton wrote alongside the image.
Comments from fans and Bachelor Nation poured in, wishing Colton well and offering their support during this difficult time.