"Far from valuing its people as equal citizens, this cruel dictatorship measures them, scores them, and ranks them based on the most arbitrary indication of their allegiance to the state," Trump said in the address.
"Those who score the highest in loyalty may live in the capital city. Those who score the lowest starve," he said in the first address to the national assembly in Seoul by a US head of state in 24 years.
The Demilitarized Zone that splits the Korean peninsula in two was, Trump said, "the line that today divides the oppressed and the free".
The North's ruling Kim family has ruled the impoverished, isolated country with an iron fist and pervasive personality cult for three generations, showing no tolerance for political dissent.
The regime has for decades been criticised for a range of rights abuses including torture, rape and execution of perceived critics or those trying to flee the country.
It is also known to operate multiple prison camps across the country where hundreds of thousands languish under forced labour and starvation.
Trump noted many cases of such abuses, including "a nine-year-old boy... imprisoned for ten years because his grandfather was accused of treason."
"The horror of life in North Korea is so complete, that citizens pay bribes to government officials to have themselves exported abroad as slaves," he said.