Philadelphia has been the scene of protests as well as riots and looting since Monday following the police shooting of a Black man, Walter Wallace Jr.
A footage, filmed by a bystander, shows police officers have fired 14 rounds at him after he ignored their commands to drop a knife.
Wallace, 27, was said to be suffering from mental health episode at the time of his death.
“The mayor or whoever it is that's allowing people to riot and loot and not stop them is also just a horrible thing,” Trump said on Wednesday.
“I saw the event. Everybody did, it was on television. It was a terrible event. I guess that's being looked at very strongly. We have the federal government looking at it also. But the rioting in Philadelphia has to stop. They have to stop it,” he added.