Pietro Senex (died 1134) was Cardinal-Bishop of Porto from 1102 until his death.
He was born probably in Rome. He appears for the first time as cardinal in March 1102, when he exercised the legatine duties in Benevento. From 1106 until 1109 he is attested as governor of Benevento. He belonged to the sixteen cardinals who confirmed the treaty of Ponte Mammolo between Pope Paschalis II and Emperor Henry V (April 1111). He participated in the Lateran council in the following year. He subscribed the papal bulls between March 8, 1114 and April 10, 1129. Papal vicar at Rome, 1117–20. He participated in the papal election, 1118, in which pope Gelasius II was elected. In March 1119 he presided over the ratification by the Roman clergy of the election of Pope Callixtus II, made in Cluny by few cardinals present at the deathbed of Gelasius II. He served as legate of Callixtus II in Venice and Outremer. During the papal election, 1124 he unsuccessfully opposed the illegal intervention of Roberto Frangipani, which resulted with the election of Pope Honorius II. He became dean of the College of Cardinals ca. 1126. In the papal election, 1130 he gave his vote to Pietro Pierleoni, who became antipope Anacletus II, and consecrated him to the episcopate on February 23, 1130. He subscribed the electoral decree of Anacletus on February 14, 1130, and the bulls issued by antipope on March 27 and April 24 of the same year. He died probably in 1134, at very advanced age, without making submission to the legitimate pope Innocent II.
- Ian Stuart Robinson (1990). The Papacy 1073–1198. Continuity and Innovation. Cambridge Medieval Textbooks. Cambridge University Press. ISBN 0-521-31922-6.
- Rudolf Hüls (1977). Kardinäle, Klerus und Kirchen Roms: 1049–1130 (in German). Bibliothek des Deutschen Historischen Instituts in Rom. ISBN 978-3-484-80071-7.
- Klewitz, Hans-Walter (1957). Reformpapsttum und Kardinalkolleg. Die Entstehung des Kardinalkollegiums. Studien über die Wiederherstellung der römischen Kirche in Süditalien durch das Reformpapsttum. Das Ende des Reformpapsttums (in German). Hermann Gentner Verlag, Darmstadt.
- Brixius, Johannes Matthias (1912). Die Mitglieder des Kardinalskollegiums von 1130-1181 (in German). Berlin: R. Trenkel.