Rishi Sunak has become the third British Prime Minister in the space of 50 days.
He was officially appointed to the role after a meeting with King Charles, in which he was asked to form a government in the King's name.
It followed a prior meeting between the King and outgoing Prime Minister Liz Truss, who formally handed in her resignation.
Mr. Sunak promised to earn 'trust' - but also acknowledged difficult decisions are going to have to be made when it comes to the economy.
Boris Johnson's congratulated his former Chancellor, urging the whole party now to back him.
Liz Truss used her farewell speech to promise 'brighter days ahead'.
The song 'I Predict A Riot' by the Kaiser Chiefs was played by protestors near the Downing Street gates as she spoke.
Rishi Sunak will now likely spend his afternoon appointing people to his Cabinet.