"Congratulations to Liz Truss on her decisive win. I know she has the right plan to tackle the cost of living crisis, unite our party and continue the great work of uniting and levelling up our country. Now is the time for all Conservatives to get behind her 100 per cent," Johnson said on Twitter.
Johnson added that he has been proud to be the leader of the Conservative Party for the past three years, as he managed to achieve Brexit, ensure the fastest COVID-19 vaccine rollout in Europe and provide "vital support to Ukraine."
Former UK Prime Minister Theresa May also congratulated Truss on her victory in the election and called on the Conservative Party to work together to solve the problems that the country faced.
"Congratulations to Liz Truss. We Conservatives must now work together to address the challenges facing our country. Tackling the cost of living, delivering for those in need and managing the public finances responsibly. I look forward to supporting the government in that task," May said on Twitter.
Earlier in the day, it was announced that UK Foreign Secretary Truss had won the Conservative Party leadership race and will become the new prime minister of the United Kingdom on Tuesday.
Johnson, who replaced May as prime minister in 2019, announced on July 7 that he was stepping down as prime minister and head of the UK Conservative Party.