A Conservative leadership election will be completed within the next week, she said. "I will remain as prime minister until a successor has been chosen," said Truss.
Liz Truss was prime minster for 45 days, the shortest-serving PM in UK history.
In her speech, Ms Truss said she entered "office at a time of great economic and international instability", as war rages in Ukraine and living costs skyrocket.
The prime minister said her government delivered on providing support for energy bills and reversing a rise in National Insurance, a tax on workers and companies.
But Ms Truss's resignation comes after a period of political and economic turbulence, which forced her government to cancel tax cuts that sent financial markets into turmoil.
