Dressed in the bright Hawaiian shirt traditionally worn at the Thai New Year festival Songkran, Prayuth showed up unannounced at Bangkok's famous Khaosan Road and was the instant target of throngs of revelers armed with buckets and water guns.
Videos on social media showed the former army chief, who seized power in a coup in 2014, wiping water constantly from his head and face and taking fire from all angles as he strolled through crowds that cheered and whistled.
The 69-year-old general went on the offensive soon after, taking aim at party goers from close range with a fluorescent pump-action water gun.
Prayuth has been struggling in opinion polls ahead of the May 14general election, with several recent national surveys showing his popularity at less than half of the front runner for the top job, Paetongtarn Shinawatra.