Mokhtar Belmokhtar was killed in the eastern city of Ajdabiya, a statement from Libya's government said.
The US says Belmokhtar was targeted and the strike was successful, but it is assessing the operation's results and would give details "as appropriate".
Mokhtar Belmokhtar's death has been reported many times in the past.
Born in Algeria, Belmokhtar was a former senior figure in al-Qaeda in the Islamic Maghreb (AQIM), but left to form his own militia.
He gained notoriety with the attack on the In Amenas gas plant in Algeria in 2013, when about 800 people were taken hostage and 40 killed, most of them foreigners, including six Britons and three Americans.
The US has filed terror charges against him and officials said they believed he remained a threat to Western interests.
"Belmokhtar has a long history of leading terrorist activities as a member of AQIM, is the operational leader of the al-Qaeda-associated al-Murabitoun organisation in north-west Africa, and maintains his personal allegiance to al-Qaeda," said Pentagon spokesman Col Steve Warren.
The Libyan government said the strike came after consultation with the US. Their statement said it resulted in the death of the "terrorist Belmokhtar".
Libya has been in chaos since the ousting of Muammar Gaddafi in 2011.
Its internationally recognised parliament is operating in exile in the eastern port of Tobruk.
A rival parliament, the Islamist-dominated General National Congress, is nearly 1,000km (620 miles) to the west in Tripoli.
Rival militia have been battling to fill the power vacuum, with Islamic State militants battling other Islamists in the east.
A top Islamist militant who ordered a deadly attack on an Algerian gas plant two years ago has been killed in a US air strike in Libya, officials say.