The Tory party leader confirmed her departure in a statement delivered outside Number 10 on Thursday.
A leadership election will take place within the next week, Ms Truss confirmed.
She will remain as prime minister until a successor has been chosen.
Ms Truss met with Sir Graham Brady, the powerful chairman of the 1922 Committee of backbenchers, to No 10 in a hastily-arranged meeting on Thursday.
Climate minister Graham Stuart said he had been “proud” to serve Liz Truss as Prime Minister.
In a statement, the junior minister said: “I am sorry to see the Prime Minister resign today.
“When she entered Downing Street, the Prime Minister set out a vision for a low tax, high growth economy to make our country an aspiration nation. During her time in office she delivered on her promise of reversing the National Insurance increase and introduced the Energy Price Guarantee Bill to give families and businesses the support they need with their energy bills.
“I have been proud to serve during her Premiership as Minister for Climate working to deliver the Government’s positive vision to achieve our aim of making the UK Net Zero by 2050.
“I will support the next leader of the Conservative Party in their work to provide security and stability for the country, and I wish the Prime Minister and her family the very best for the future.”