Ranuccio I Farnese (28 March 1569 – 5 March 1622) reigned as Duke of Parma, Piancenza and Castro from 1592. A firm believer in absolute monarchy, Ranuccio, in 1594, centralised the administration of Parma and Piacenza, thus rescinding the nobles' hitherto vast prerogative. He is best remembered for the "Great Justice" of 1612, which saw the executions of many Piacentine nobles suspected of plotting against him. Because one of the conspirators, Gianfrancesco Sanvitale, falsely implicated several Italian monarchs, namely Vincenzo Gonzaga, Duke of Mantua, and Cesare d'Este, Duke of Modena, in the plot, Vincenzo and Cesares' names appeared on the list of conspirators during formal court proceedings; as a result, Ranuccio's reputation among the monarchs of Italy was irreparably damaged because it was evident that he gave credence to Gianfrancesco's obviously false confession. When, consequently, in the early 1620s, Ranuccio was scouting for a bride for his younger legitimate son and heir, Odoardo, none of the Italian ruling families were forthcoming with princesses. He did, however, manage to engineer a match with Margherita de' Medici, daughter of Cosimo II of Tuscany.

The husband of Margherita Aldobrandini, niece of Pope Clement VIII, Ranuccio, the son of a Portuguese infanta, was considered as a potential monarch of Portugal when his childless great - uncle King Henry I died. The throne, however, passed to Philip II of Spain, whose troops had promptly occupied the country after Henry I's death.

His great - uncle Henry I of Portugal's death triggered the struggle for the throne of Portugal when Ranuccio was 11 years old. As the son of the late elder daughter of Infante Edward, 4th Duke of Guimarães, the only son of King Manuel I whose legitimate descendants survived at that time, Ranuccio was according to the feudal custom the first heir to the throne of Portugal. However his father was an ally and even a subject of the Spanish king, another contender, so Ranuccio's rights were not very forcibly claimed. Instead, Ranuccio's mother's younger sister Catherine, Duchess of Braganza, claimed the throne, very ambitiously, but failed.

Under Ranuccio I's rule, the dukedom annexed the territories of Colorno, Sala Baganza, and Montechiarugolo. He guided a cultural renewal in the city of Parma, supporting the arts and constructing the 4,500 seat Farnese Theater. Numerous improvements and monuments in Parma were constructed under Ranuccio I at his behest, including a revitalization of the University of Parma and the final expansion of the city walls. Construction of the Palazzo della Pilotta, the court palace of the Farnese family, was completed in 1620.

Ranuccio I is also remembered for his cruelty, one infamous example being the public execution of over 100 Parma residents suspected of conspiring against him.

He was succeeded by his son Odoardo, initially under the regency of Ranuccio's brother, Odoardo Farnese.

He married Donna Margarita Aldobrandini daughter of Don Giovanni Francesco Aldobrandini, Prince of Carpineto and his wife the heiress Donna Olimpia Aldobrandini, Princess Campinelli on 7 May 1600 in Rome, St. Sixtus. The couple had nine children.