In a televised address from the White House, the American leader said he would be visiting the city on Wednesday “on a very, very sad moment for me ... for everybody no matter where you are, no matter what your thought process”.
In a slow and sombre statement, Mr Trump said the nation was “joined together in sadness, shock and grief” but that the US would rally together.
“Our unity cannot be shattered by evil, our bonds cannot be broken by violence,” he said. “We call upon the bonds that unite us: our faith, our family, and our shared values. We call upon the bonds of citizenship, the ties of community, and the comfort of our common humanity.”
Mr Trump did not describe the gunman or suggest any possible motivation or affiliation. He praised the first responders whom he said had prevented further loss of life.
He also offered condolences to the families of those killed, saying: “We cannot fathom their pain. We cannot imagine their loss.
“We are praying for you. We are here for you.”
He also ordered that the American flag at the White House and at all public buildings across the nation be flown at half-mast.
Mr Trump was speaking hours after 64-year-old Stephen Paddock unleashed a hail of bullets from the 32nd floor of the Mandalay Bay Hotel, on the Las Vegas strip, onto an outdoor country music festival below. The gunman killed at least 58 and injured more than 500 others before taking his own life.
The president, who quoted from the Bible and invoked God several times, said he prayed for the day when the “innocent are safe from hatred and from fear”.
“At times such as these I know we are searching for some type of meaning in the chaos, some kind of light in the darkness. The answers will not come easy,” the president said.
Unlike his predecessor Barack Obama, who used mass shootings to call for stricter gun control measures, Mr Trump made no mention of firearms restrictions.
During the presidential campaign, the president cast himself as an ardent protector of the Second Amendment and proclaimed that if more “good guys” were armed with firearms there would be fewer gun tragedies. After the Orlando nightclub shooting last year, he suggested that if the club were not a gun-free zone, someone would have been able to stop the bloodshed.
“I was obviously talking about additional guards or employees,” he tweeted later.
Trump has long-standing connections to Las Vegas. He owns a hotel just off the strip, about three miles from the shooting site, and has been supported by some of its biggest casino moguls, including Phil Ruffin and Sheldon Adelson.