President Joe Biden has said that a new $95 billion package to support countries including Ukraine and Israel will help “make the world safer”.
The signing of the bill, which also includes assistance for Taiwan, ends months of wrangling with Republicans in Congress.
Mr Biden said the bill would “give vital support to America's partners so they can defend themselves from threats to their sovereignty”.
He added: “I am making sure that the shipments start right away. In the next few hours we are going to begin sending equipment to Ukraine, for air defence munitions, for artillery, for rocket systems and armoured vehicles.”
The bill includes $61 billion in aid to Ukraine and $26 billion for Israel, as well as $1 billion in humanitarian assistance to Gaza and $8 billion to counter China's military might. It passed the Senate in a bipartisan vote of 79-18.
The initial aid package includes vehicles, Stinger air defense munitions, additional ammunition for high-mobility artillery rocket systems, 155mm artillery ammunition, TOW and Javelin anti-tank munitions and other weapons that can immediately be put to use on the battlefield.
Ukraine had been begging for the long-range system after suffering a number of setbacks on the battlefield.
However, the bill faced months of opposition from the Republican right, who have expressed scepticism over America’s involvement in overseas conflicts.
Mr Biden said the passage of the legislation would send a direct message to Russian President Vladimir Putin, who launched an invasion of Ukraine in February 2022.
"He's failed again. American stands with our friends. ... We bow to no one, no one, certainly not Vladimir Putin," he said.
“This is consequential … Had we failed to step up, lord only knows what would happen to the cohesion in Nato.”
Ukrainian President Volodymyr Zelensky said the legislation "reinforces America's role as a beacon of democracy and leader of the free world".
Democratic Senate Majority Leader Chuck Schumer said: "After more than six months of hard work and many twists and turns in the road, America sends a message to the entire world: we will not turn our back on you."
The President’s likely opponent in the November presidential election, Donald Trump, has objected to the aid.
Mr Trump has previously said that he would let Nato allies fend for themselves if they did not increase their defence spending.
American officials said on Wednesday that Ukrainian forces had already used long-range ballistic missiles secretly provided by the US to attack Russian forces.
The weapons were used to bomb a Russian military airfield in Crimea last week and Russian forces in another occupied area overnight.
The new missiles give Ukraine nearly double the striking power that it had with the mid-range version of the weapon that it received from the US last October.
