Thousands watched in St Peter’s Square as Pope Francis held the canonisation mass for the nun, revered as one of the icons of the 20th century.
The service took place the day before the 19th anniversary of her death.
The pontiff said in Latin: “For the honour of the Blessed Trinity… we declare and define Blessed Teresa of Calcutta (Kolkata) to be a Saint and we enroll her among the Saints, decreeing that she is to be venerated as such by the whole Church.”
People started clapping and cheering even before he had finished speaking.
Among the audience were hundreds of sisters from the Missionaries of Charity, the mission that Mother Teresa founded, with their trademark blue-trimmed saris.
Joining them were 1,500 homeless people and 13 heads of state or government.
The homeless people will be offered a lunch of pizza after the canonisation ceremony is completed, in honor of her work.
Saint Teresa of Kolkata, as she will now be known, founded and ran a mission in what was then Calcutta, one of India’s largest cities where levels of deprivation were high.