The Treasury Department's waiver lets countries purchase Russian oil and petroleum products loaded on vessels as of Friday through to May 16.
It replaces a 30-day waiver that expired on April 11 and excludes transactions involving Iran, Cuba and North Korea.
The move is part of the administration's effort to control global energy prices that have shot higher during the US-Israeli war with Iran.
It came after countries in Asia, suffering from the global energy shock, pressed Washington to allow alternative supplies to reach markets.
"As negotiations (with Iran) accelerate, Treasury wants to ensure oil is available to those who need it," a Treasury Department spokesperson said.
Just two days earlier, Treasury Secretary Scott Bessent said Washington would not be renewing the waiver for Russian oil and another for Iranian oil, which is set to expire on Sunday.
Global oil prices tumbled 9 per cent on Friday to about $90 a barrel after Iran temporarily reopened the Strait of Hormuz, an oil choke point in the Gulf.
But the war has already created the worst global energy supply disruption in history, the International Energy Agency has said.
The war, which enters its eighth week on Saturday, has damaged more than 80 oil and gas facilities in the Middle East, and Tehran has warned it could close the strait again if the recent US Navy blockade of Iranian ports continued.
High oil prices are a threat to President Donald Trump's fellow Republicans ahead of November's midterm elections.
Trump has also faced pressure from partner countries on the oil price.
A US source said partner countries on the sidelines of Group of 20, World Bank and International Monetary Fund meetings in Washington this week had requested the US extend the waiver.
And he spoke about oil this week in a call with Prime Minister Narendra Modi of India, a big purchaser of Russian oil.
The waiver on Iranian oil, which the Treasury Department issued on March 20, allowed about 140 million barrels of oil to reach global markets and helped relieve pressure on energy supply, Bessent said last month.
US lawmakers from both political parties had slammed the administration over the sanctions waivers, saying they stood to help the economy of Iran while it was at war with the US and of Russia as it was at war with Ukraine.
The waivers could impede the West's efforts to deprive Russia of revenue for its war in Ukraine and put Washington at odds with its allies.
European Commission President Ursula von der Leyen has said now is not the time to relax sanctions against Russia.
Russian President Vladimir Putin's special envoy Kirill Dmitriev said an extension of the US waiver will affect another 100 million barrels of Russian oil, bringing the total volume affected by both waivers to 200 million barrels.
Dmitriev, who travelled to the US on April 9 for meetings with members of the Trump administration ahead of the previous waiver expiry, said on his Telegram channel that the extension faced "active political opposition."
