Michael Phelps just shared some heartbreaking news ... announcing his father, Fred, has passed away.
The Olympics superstar provided the update via Instagram on Monday ... saying, "You’ll always be my dad ... And I’ll always be your son ... Love you dad and I will miss youu2764ufe0fu2764ufe0f rip dad."
37-year-old Phelps also posted several throwback pictures of his father ... including several snaps with his grandchildren.
Just like his son, Fred was an athlete -- he played college football and once tried out for the Washington Commanders in the 1970s.