Brook is widely regarded as one of the finest young batters in world cricket.
Brook, who is widely regarded as one of the finest young batters in world cricket, failed to make a cut in England’s provisional World Cup squad but might travel as a reserve.
Speaking to Sky Sports, Pietersen said he was stunned after receiving the news because he held Brook in high regard.
"I'm absolutely flabbergasted that [Brook] cannot make England's World Cup squad because he is pure quality. I just cannot believe it," Pietersen stated.
"I saw this guy at Headingley two years ago when The Hundred took off and I immediately saw a superstar. I saw someone playing 360, who could hit the same balls over extra-cover or midwicket," he added.
On Wednesday (August 16), England unveiled their provisional squad for the ODI World Cup, which also saw Ben Stokes making a U-turn on his ODI retirement announced.
While Brook was left disappointed by the snub, the right-hander said he had to move on.
"Obviously it's disappointing but I can't do anything about it now," Brook was quoted as saying by Sky Sports.
"You've just got to move on. I'm trying not to think about it anymore. [Stokes] is one of the best players to ever play cricket, so I can't really complain, can I?" he further said.
While Harry Brook has played only three ODIs for England so far, he has established himself as a key player in the Test side. In 12 Tests, he has amassed 1181 runs at an average of 62.16 and a strike rate of 91.76 with four centuries.
"I feel like I'm playing well at the minute and feel I could potentially add value to the team. [But] there's always something more you could do," Brook said after his ODI World Cup omission.
"Although I've played a lot of T20 cricket, I don't know if I've done as well as I have in the past in the last six months, so that might have had an effect," he added.
England Squad
Jos Buttler (captain), Moeen Ali, Gus Atkinson, Jonny Bairstow, Sam Curran, Liam Livingstone, Dawid Malan, Adil Rashid, Joe Root, Jason Roy, Ben Stokes, Reece Topley, David Willey, Mark Wood, Chris Woakes.