Saint Malachy is believed you've written it.. Some have said he didn't and instead the document is a fraud.
The 1913 Catholic Encyclopedia wrote this on Malachy:
The most famous and best known prophecies about the popes are those attributed to St. Malachy. In 1139 he went to Rome to give an account of the affairs of his diocese to the pope, Innocent II, who promised him two palliums for the metropolitan Sees of Armagh and Cashel. While at Rome, he received (according to the Abbé Cucherat) the strange vision of the future wherein was unfolded before his mind the long list of illustrious pontiffs who were to rule the Church until the end of time.
This full prophecy is available here to read.
Malachy (supposedly) wrote this on the next Pope:
In persecutione extrema S.R.E. sedebit Petrus Romanus, qui pascet oves in multis tribulationibus: quibus transactis civitas septicollis diruetur, & Judex tremêdus judicabit populum suum. Finis.
(In extreme persecution, the seat of the Holy Roman Church will be occupied by Peter the Roman, who will feed the sheep through many tribulations, at the term of which the city of seven hills will be destroyed, and the formidable Judge will judge his people. The End.)