Trump's tweet also pointed to Vietnam as an example that North Korea could follow.
"Vietnam is thriving like few places on earth. North Korea would be the same, and very quickly, if it would denuclearize. The potential is AWESOME, a great opportunity, like almost none other in history, for my friend Kim Jong Un. We will know fairly soon - Very Interesting!" the president said.
Trump, at a meeting with Vietnamese President Nguyen Phu Trong, repeated that Vietnam is an example for North Korea.
The American president said he and Kim "felt very good about having this very important summit in Vietnam" because the Southeast Asian country is "an example as to what can happen with good thinking."
If North Korea follows in Vietnam's footsteps, the rogue nation's gradual opening up could bring in investment opportunities worth up to $9 billion per year, Morgan Stanley estimated.
"North Korea's 18 million working age population would join Asia's production supply chain at an hourly wage cost lower than Vietnam's," the bank said in a research note. "A liberalised North Korea would provide the missing link in improving the trade connectivity of the Korean peninsula to Europe if inter-Korea rail connects to Russia and China."
Vietnam's economy has thrived at a time when global activity slowed down. The Southeast Asian frontier market reported growth of 7.1 percent last year — its strongest pace in a decade and also among the fastest in the world. The country was also frequently cited for its potential to be a beneficiary of the U.S.-China trade war.
But Vietnam has relied mostly on labor-intensive manufacturing to power its growth over the last three decades, helped by a young workforce and low wages. Productivity growth and technological adoption have not kept up, and some economists have raised concerns about the country's economic future once those advantages fade.
Trump and Kim are scheduled to meet in the Vietnamese capital city of Hanoi this week, less than a year after the two leaders met in Singapore. Last June's summit was the first in-person meeting between a sitting U.S. president and a North Korean head of state.
The two leaders are expected to discuss denuclearizing the Korean peninsular, although the two sides appear far apart on the idea of North Korea getting rid of its nuclear weapons. That has long been a U.S. precondition for economic sanctions to be lifted, but analysts say Kim is unlikely to relinquish North Korea's hard-won status as a nuclear power.
— Reuters contributed to this report.