Speaking from Downing Street, he thanked the millions of people who voted for him and said he had fought so long to remain at Number 10 because "I thought it was my job, my duty and my obligation to you".
Mr Johnson added it was "painful not to see it through" but added: "No one in politics is remotely indispensable."
There had been intense pressure on the PM to quit after more than 50 resignations from the government payroll and waves of backbenchers appealing for him to go.
The mass rebellion began on Tuesday after Downing Street admitted the PM knew about allegations of inappropriate behaviour against disgraced former Deputy Chief Whip Chris Pincher in 2019 but still appointed him in February and sent ministers out to defend him.
Health Secretary Sajid Javid and Chancellor Rishi Sunak were the first to resign on Tuesday night, but over the past 48 hours, MPs from all levels of government piled in their letters and demanded him to go.
