Speaking to journalists at Joint Base Andrews military airfield near Washington, on Sunday, Trump described the deal as a business transaction.
"The European Union is paying for it. We're not paying anything for it, but we will send it."
The US president has grown increasingly disenchanted with Putin because the Russian leader has resisted Trump's attempts to negotiate a ceasefire between Ukraine and Russia.
Ukrainian President Volodymyr Zelenskiy has asked for more defensive capabilities to fend off a daily barrage of missile and drone attacks from Russia.
"We will send them Patriots, which they desperately need, because Putin really surprised a lot of people," Trump said.
"He talks nice and then bombs everybody in the evening. But there's a little bit of a problem there. I don't like it," Trump said.
"We basically are going to send them various pieces of very sophisticated military equipment. They are going to pay us 100 per cent for that, and that's the way we want it," Trump said.
He plans to meet NATO Secretary General Mark Rutte to discuss Ukraine and other issues this week.
Ukrainian President Volodymyr Zelenskiy has been desperate for more robust weapons to shoot down incoming Russian fire.
He has accused Russia of increasingly terrorising his country's civilians, reporting that at least 1800 drones were launched at Ukraine in the past week.
Russian forces also dropped more than 1200 glide bombs on Ukraine this week and launched 83 missiles, the president said on Telegram on Sunday.
"The Russians are increasing the terror against cities and municipalities, to intimidate our people even further," Zelenskiy wrote.
The Ukrainian leader meanwhile praised the country's air defence.
Specially developed interceptor drones shot down hundreds of Iranian-made Shahed combat drones launched at Ukraine this week, he said.
Ukraine has been fending off a full-scale Russian invasion for more than three years, in large parts thanks to foreign weapons.
Meanwhile, NATO Secretary General Mark Rutte is set to meet Trump this week.
NATO in a statement said Rutte would be in Washington DC on Monday and Tuesday and would meet with Trump, Secretary of State Marco Rubio, and Defense Secretary Pete Hegseth as well as Congress.
with dpa
