Sean Ono Lennon -- John and Yoko Ono's only kid -- says he's cracked open the Duke of Sussex's memoir, a year after its release, and he was left underwhelmed by the tell-all, writing ... "My long awaited review of Prince Harry’s autobiography is two words. 'Spare Me.'"

As fans of the prince hit back at Sean's criticism in his comments, the musician doubled down, calling the prince "a buffoon" and an "idiot" in replies to various people online.

While Sean admitted to having empathy for PH, he blasted the royal for "the way he whines and wangs on about things" ... feeling particularly unimpressed with Harry's tidbit about his frostbitten penis.

However, he clarified his scathing review a bit -- later saying it was just a bit of fun, but in the same breath telling his followers King Charles' son "deserves to be mocked."

Sean isn't the first celeb to speak out about Harry's memoir ... although most weighed in on the book back in January 2023 -- when it hit shelves, and most stars were easy on him. For instance, Stephen Colbert praised the book, calling it a good read.

On the flip side, however, some have been harsher (like Sean here) -- Sharon Osbourne blasted Harry, accusing him of turning the royal family into a soap opera of sorts.