Legendary swimmer Michael Phelps is grieving over the death of Brian Campbell ... sharing a heartfelt tribute to the longtime Olympic swimming staffer.
Team USA Swimming announced Campbell's passing on Monday ... saying the massage therapist -- who also worked for UCLA -- died at the age of 68.
"He served American swimmers for decades on the pool decks of numerous Olympic Games and World Championships," the organization said.
"We send our gratitude to Brian for his years of service to our sport, and our thoughts to the friends, family, and athletes who knew him."
Several swimmers were devastated by the news ... including Phelps, who said, "So many memories Brian... rip buddy."
"Thought and prayers to ur fam."
Phelps -- the most decorated Olympian of all time -- went on to reminisce on their conversations over the 20 years they were together ... adding, "thank you for being such a good human!"
Phelps' former coach Bob Bowman also paid tribute ... praising Campbell for his energy.
"Rest easy Brian," Bowman said. "We will all miss your smile and positivity. Love you my friend. 💔"