Conor gave a heartfelt goodbye to his four-legged friend minutes ago ... saying, "So heart broken to say my dog Hugo has passed away."

McGregor reflected on the memories he shared with the mixed-breed for so many years ... dating back to the days before he became a household name in the MMA community.

"He was with me all the way, my closest companion. From sleeping on a bed frame without a mattress to everything else that came after. Thank you for all the work put in alongside me to get this life correct for our family."

Conor shared several pictures with Hugo on Instagram on Wednesday ... showing them side-by-side as he trained, relaxed and enjoyed his downtime.

"All the love and cuddles we will miss forever and ever our Hugo! Gutted 💔"