Placido Titi (1603–1668) was an Italian Christian monk and astronomer of the Olivetan order, professor of mathematics, physics and astronomy at the University of Pavia from 1657 until his death.
He lived and worked in Milan, that at that time was ruled by the family Habsburgs of Spain. He worked especially for the Archduke Leopold Wilhelm of Austria, who had a strong interest in the science of alchemy and astrology, and Placido (Placid) Titi dedicated his studies about astrological houses to him.
Placido Titi made the domification (house) system famous, known as the Placidian system, which is nowadays used by modern astrology. He did not invent the method, but he learned it from the Jewish astrologer Abraham Ibn Ezra as the system employed by Ptolemy, an attribution which was accepted by Placidus.