Kim Jong Un's regime "has interpreted America's past restraint as weakness," he said. "This would be a fatal miscalculation. This is a very different administration than the United States has had in the past."
"Do not underestimate us, and do not try us," Trump warned, urging the pariah state to stop developing ballistic missiles and calling for it to dismantle its nuclear program.
"The weapons you are acquiring are not making you safer, they are putting your regime in grave danger," the U.S. leader said in remarks directed to Kim. "Every step you take down this dark path increases the peril you face."
Trump is the first U.S. president to address South Korea's parliament, known as the National Assembly, since Bill Clinton in 1993. The U.S. leader is on a landmark Asia tour and arrived in Seoul on Tuesday following a two-day stay in Japan. He is scheduled to depart for China later on Wednesday.
Government officials in Asia's fourth-largest economy were reportedly anxious about Trump's remarks, fearful that he could increase tensions with Pyongyang, local media said last week.
"We will not permit America, or our allies, to be blackmailed or attacked. We will not allow American cities to be threatened with destruction," Trump warned on Wednesday, referring to Kim's repeated threats of launching an intercontinental ballistic missile toward the world's largest economy.
"The longer we wait, the greater the danger grows, and the fewer the options become. To those nations that choose to ignore this threat, or worse still, to enable it, the weight of this crisis is on your conscious."
More generally, Washington was helping South Korea "far beyond what any other country has ever done," Trump said, adding that both nations "will work things out far better than anybody understands or even appreciates."
He also praised Seoul's economic recovery following the Korean War, saying "the more successful South Korea becomes, the more decisively you discredit the dark fantasy at the heart of the Kim regime."
"South Korea will never allow what's going on in North Korea to continue to happen," he stated.
Trump also took the opportunity to commend the U.S. economy, Washington's battle against the Islamic State and the power of the U.S. army, noting that Washington had three aircraft carriers and a submarine in the Pacific. "I want peace through strength," he said.
Earlier in the day, Trump and his team attempted a trip to the heavily fortified demilitarized zone that separates North and South Korea, according to NBC News. But the trip was cancelled due to bad weather.
On Tuesday, the commander-in-chief signaled he was ready to negotiate with Kim. "It really makes sense for North Korea to come to the table and make a deal," he said at an earlier news conference with South Korean President Moon Jae-in.
Some in South Korea fear that a major provocation from Kim's regime could result in Washington staging unilateral military operations. Moon, who advocates engagement with Pyongyang instead of the hard-line stance shared by Tokyo and Washington, has warned that "no one should be allowed to decide on a military action on the Korean Peninsula without South Korean agreement."
Analysts had warned it was essential for both Trump and Moon to present a united front on tackling the North. The two allies share fundamental differences on strategies, but they remain on the same page on the underlying goal of denuclearization. Trump has previously criticized Moon for the latter's outreach to Pyongyang, but on Tuesday he praised the South Korean leader for "great cooperation."