Trump's almost admiring remarks came amid soaring tensions with North Korea over its missile and nuclear programs, with an alarmed Washington looking to China for help in reining in Kim.
Trump said he had "no idea" whether Kim was sane or not, but said the North Korean leader had faced a formidable challenge in taking over the country at a reported age of 27 after his father's death in 2011.
"He's dealing with obviously very tough people, in particular the generals and others. And at a very young age, he was able to assume power," Trump said in the interview with CBS's "Face the Nation."
"A lot of people, I'm sure, tried to take that power away, whether it was his uncle or anybody else. And he was able to do it.
"So obviously, he's a pretty smart cookie," he said.
"But we have a situation that we just cannot let -- we cannot let what's been going on for a long period of years continue," Trump added.
North Korea has kept the West on edge for weeks over signs it may conduct a sixth nuclear test, punctuated by a series of missile tests that have aroused US fears that the regime may be close to developing a ballistic missile capable of hitting the US mainland with a nuclear warhead.
