Bethell, 22, has not yet featured for Royal Challengers Bengaluru during the current IPL season, prompting Cook to suggest the batter would be better served returning to England to play county cricket for Warwickshire.
Pietersen, who has previously played for the Bengaluru franchise, dismissed that view on Thursday, arguing that exposure to the IPL environment remains valuable even without immediate game time.
"Alastair Cook has absolutely NO IDEA what it's like to be in the IPL, what it's like to always be around the best players in the world," former England captain Pietersen said in an X post.
"So his opinion on Jacob Bethell doesn't matter at all. Stay in India, Jacob. I know, even though you're not playing, you're learning and will be a way better player."
Bethell responded to Cook's comments on Wednesday, defending his decision to stay with Bengaluru and describing the benefits of being available in the tournament.
"It's something that not many people will understand how cool it is until actually being around a team or the tournament itself when you're here," Bethell told Sky Sports.
"It just has a completely different feel. It feels like everyone almost ups their game subconsciously without even really knowing because of the calibre of the tournament.
"I feel like that even happens when you're not on the starting XI or even starting XII as it might be now with the impact player."
Bethell has emerged as one of England’s brightest young batters and is increasingly viewed as a long-term option in the test top three.
His knock of 154 in the final Ashes test in Sydney in January was one of the few positives in England’s 4-1 series defeat in Australia and strengthened his credentials as a player capable of batting early against high-quality bowling attacks.
"For that top-order batting, the way he played at Sydney, against that attack, in those conditions, I've looked at a player there and I'm certain this bloke can open. If he can bat three, he can open," Cook said.