"Delivering these tanks to the field is going to take time. Time that we'll see and we'll use to make sure the Ukrainians are fully prepared," he said. Senior US officials earlier said it would take months, not weeks.
Announcing the decision in remarks at the White House, Biden said the move was not "an offensive threat to Russia."
He said the Abrams tanks were needed to help Ukrainian forces "improve their ability to maneuver in open terrain."
Anatoly Antonov, Moscow's ambassador to Washington, earlier warned that a US approval for Abrams deliveries would be "another blatant provocation against the Russian Federation."
The announcement came hours after Germany agreed to deliver Leopard 2 battle tanks to Ukraine.
"Germany has really stepped up," Biden said, adding that US and European allies were united in their efforts to support Ukraine.
"The expectation on the part of Russia is we're going to break up," he said. "But we are fully, totally and thoroughly united."
The US was not initially keen to send some of its most powerful battle tanks to Ukraine, arguing that the Abrams were too difficult to operate and maintain.
But senior officials said Washington had to reverse course to persuade Berlin to send its more easily used Leopard 2 tanks to Ukraine.
Answering a reporter's question on whether Berlin "forced" Biden to change his mind, the US president said: "Germany didn't force me to change my mind, we wanted to make sure we were all together."
Separately, the German chancellery said Olaf Scholz spoke on the phone with Biden, French President Emmanuel Macron, British Prime Minister Rishi Sunak and Italian Prime Minister Giorgia Meloni on Wednesday afternoon.
The leaders agreed to continue military support to Ukraine in close Euro-Atlantic coordination, Scholz's office said.
Ukraine hails 'historic day'
Ukrainian President Volodymyr Zelenskyy thanked Biden for the "powerful" decision. "It's an important step on the path to victory," Zelenskyy wrote on Twitter.
Andriy Yermak, the head of the Ukrainian presidential office, called it a "historic day" that would help determine the outcome of the war.
"The main thing is that this is only the beginning. We need hundreds of tanks," he said in a post on Telegram.
European allies have agreed to send enough tanks to equip two Ukrainian tank battalions, or a total of 62 tanks, Biden said.
A number of European countries, including Finland, Norway and Poland, have said they would send their tanks to Ukraine.
fb/nm (AFP, AP, dpa, Reuters)