The nun is taking over the position of head of the Vatican office responsible for migrants, the environment and development, following Canadian Cardinal Michael Czerny.
Sister Alessandra is the first woman appointed to a senior position by Pope Leo.
He thus appears to be following in the footsteps of his predecessor, Pope Francis, who promoted women to top positions within the Holy See in an attempt to appease critics who said women were not given responsibility or decision-making roles within the church.
Women within the Catholic Church have long complained that they are not allowed into the higher ranks, but are instead allowed to devote themselves to healthcare, running schools and ensuring that the faith is passed on to younger generations.
