Truss, who currently serves as foreign minister, will be the U.K.'s new prime minister, after the Conservative Party named her as its next leader on Monday, replacing the outgoing Boris Johnson.
The victory may be propelled by what is seen as a wave of sympathy votes in a country still reeling from the shock of the former prime minister's assassination.
His resignation will trigger an internal election to pick a new leader of the Conservative Party, who will also be the next prime minister. He steps down after defections left him unable to govern.
Johnson was booed during the queen's Platinum Jubilee — the latest sign of intense public anger over his flouting of his own government's COVID-19 rules.
Australians awoke on Sunday to a new prime minister in Anthony Albanese, the center-left Labor Party leader, though it remains unclear whether his party can form a majority government.