Truss said she could not fulfil the mandate on which she was elected as Conservative leader and told the King about her intention to step down.
There will be a Tory leadership election to be conducted within a week, she said.
“I will remain prime minister until a successor is elected,” she said.
Her departure after 45 days in office makes her the shortest prime minister in British history.
Truss’ premiership has been in turmoil since her mini-budget last month shook markets and was later ditched by his new chancellor.
The resignation of Home Secretary on Wednesday and a chaotic vote in the House of Commons sealed her fate.