"I, for one, am glad to see he is back, and well!" Mr Trump tweeted, after the North Korean leader reportedly attended the opening of a fertiliser plant.
It was Mr Kim's first public appearance in almost three weeks.
His absence - particularly from his late grandfather's birthday celebration on 15 April - had sparked intense global speculation over his health.
KCNA news agency reported that Mr Kim cut the ribbon at the opening of the fertiliser factory on Friday, and added that crowds "broke into thunderous cheers of hurrah" when he appeared.
On Monday, amid speculation and rumour about Mr Kim's health, President Trump had said he had a "very good idea" about Mr Kim's condition, but added that "I can't talk about it".
"I just wish him well," he added at the time.