In the Oval Office at an announcement with NATO Secretary General Mark Rutte, Trump told reporters he had become deeply frustrated with Russian President Vladimir Putin.
He said Putin is pleasant to speak with on the phone but then turns around and unleashes withering bombing raids on Ukraine.
"I don't want to say he's an assassin but he's a tough guy," Trump said, noting that several of his predecessors had also become disillusioned with Putin.
The decision by Trump to send arms to Ukraine represented a turning point for the Republican president, who has spent the early months of his term trying to coax Putin into a ceasefire agreement only to be turned down every time.
Under the deal, Trump said the United States will supply weapons that will be paid for by NATO countries.
Rutte said massive numbers of weapons would be sent, including missiles, as part of a first wave of equipment.
Trump said the "top of the line" equipment, including Patriot missile systems and batteries, would be arriving very soon.
"We're going to have some come very soon, within days ... a couple of the countries that have Patriots are going to swap over and will replace the Patriots with the ones they have," he said.
Ukraine President Volodymyr Zelenskiy has made regular appeals to the US and its NATO military alliance allies for weapons to help defend itself in the grinding war.
Trump coupled his arms announcement with a vow to impose tariffs and sanctions on Russia if it will not make a ceasefire agreement within 50 days.
US Congressional leaders have been working on a Russia sanctions package.
A White House official said Trump's intent is to impose "100 per cent tariffs on Russia" and secondary sanctions on other countries that buy oil from Russia if a deal is not struck in 50 days.
"If I was Vladimir Putin today, and you're speaking about what you are planning to do in 50 days ... I would reconsider whether I should not take negotiations about Ukraine more seriously," Rutte said.
Trump has been reluctant to punish Russia but he came away from a recent phone call with Putin disappointed that the Russian leader appeared prepared to continue the war.
Rutte said Germany, Finland, Denmark, Sweden, Norway, the Netherlands and Canada all want to be a part of rearming Ukraine.
"They all want to be part of this. And this is only the first wave. There will be more. So what we will do is work through the NATO systems to make sure that we know what Ukrainians need to make packages," he said.
Trump's promise that NATO would pay for the weapons satisfies his demand that the US not continue bankrolling the war and puts the financial burden on US allies in Europe.
Zelenskiy on Monday asked First Deputy Prime Minister Yulia Svyrydenko to lead a new government, setting the stage for a political reshuffle in Ukraine.
The nomination, which requires parliamentary approval, comes as diplomatic efforts to end the war, now in its fourth year, have stalled and as Ukraine seeks to revive its cash-strapped economy and build up a domestic arms industry.
"We ... discussed concrete measures to boost Ukraine's economic potential, expand support programs for Ukrainians, and scale up our domestic weapons production," Zelenskiy wrote on X.
"In pursuit of this goal, we are initiating a transformation of the executive branch in Ukraine," he said, adding that he had proposed that Svyrydenko lead the government and "significantly renew its work".
Svyrydenko, 39, is an economist by training and has served as first deputy prime minister since 2021.
