second duke of Florence (1537–74) and first grand duke of Tuscany (1569–74).

Cosimo was the great-great-grandson of Lorenzo the Elder, the son of Giovanni di Bicci and brother of Cosimo the Elder, and was thus a member of a branch of the Medici family that had taken an active part in Medici affairs but had played no political role. Nevertheless, when he heard of the assassination of his distant cousin, Alessandro, duke of Florence, he immediately made for Florence. There, in January 1537, Cosimo was elected head of the republic, in the government of which he was to be assisted by the senate, the assembly, and the council. This election was approved by the Holy Roman emperor, Charles V, and on August 2 the emperor's general Alessandro Vitelli, at Montemurlo, defeated an army that a band of exiles had raised against Cosimo. Cosimo then had the principal captives beheaded and began, with Charles V's approval (September 1537), to style himself duke. The Florentine senate, assembly, and council were soon powerless.

Cosimo married Eleonora de Toledo in 1539. As the emperor's protégé, he was able to withstand the hostility of Pope Paul III and Francis I of France. He was shrewd and unscrupulous, and, with Florence under his control, he turned his ambition to territorial aggrandizement. His plans for annexing Lucca and Piombino in the 1540s were frustrated, but his enterprise against the republic of Siena, which sheltered exiles from Florence and pursued a pro-French policy, was successful. Cosimo launched an attack on Siena in 1554; a French army under Piero Strozzi was defeated at Scannagallo, near Marciano; and in 1555, after a long siege, the city capitulated. Philip II of Spain, as the successor of Charles V in Italy, had to agree to enfeoff Cosimo with the lordship of Siena in July 1557. The accession of Pius IV to the papacy in 1559 strengthened Cosimo still further, since Pius was a Medici of Milan and was well disposed toward the Florentine Medici. He gave a cardinal's hat to Cosimo's son Giovanni in 1560 and, after Giovanni's death, one to another son, Ferdinand, in 1563.

Having brought nearly all Tuscany under his control, Cosimo used his despotic power to promote the country's well-being. His passion for efficiency inspired him with the idea, extremely advanced for the times, of uniting all public services into a single building, the Uffizi (“Offices”), which was built for him according to Giorgio Vasari's (Vasari, Giorgio) grandiose yet practical design. In order to satisfy his taste or, better said, his Medici passion for buildings, he made Vasari his superintendent of buildings and had him redecorate the interior of the Palazzo Vecchio. He then adopted as his residence the Pitti Palace, which Eleonora had purchased unfinished in 1549. Here he entrusted the extensive work of enlargement to the architect and sculptor Bartolomeo Ammannati (Ammannati, Bartolommeo). In 1564 Cosimo and Vasari boldly built the gallery that permits convenient passage from one palace to the other by utilizing the Ponte Vecchio. Behind the Pitti Palace, the vast expanses of the hill of Boboli enabled Cosimo to indulge still another of his hereditary passions in designing, with Tribolo's help, the plan of the famous gardens.

Yet in his patronage of the arts, Cosimo was increasingly frustrated, for the great period of the officina, the workshop of Florentine masterpieces, was drawing to its close. Michelangelo could no longer be induced to stay on. In 1534 he departed for Rome, leaving the Sagrestia Nuova tombs and the Laurentian Library unfinished. But Cosimo had the artist's body brought back in 1564 and buried it himself with great pomp at Santa Croce. On the other hand, he was able to retain Jacopo Pontormo (Pontormo, Jacopo da) and Bronzino, the official court portraitists, and Ammannati, who was also an engineer and who had rebuilt the bridge of Santa Trinità after the disastrous flood of 1557. Cosimo, an archaeologist by temperament, was a true forerunner in this field. He opened up excavations on Etruscan sites from which such world-renowned pieces of ancient statuary as the “Orator” and the “Chimera” were taken. Finally, he established the Florentine Academy, which engaged in serious linguistic studies.

Cosimo was deeply afflicted when his wife, two of his daughters, and two of his sons all died within six years (1557–62); his enemies exploited these misfortunes to spread calumnies against the dynasty. On March 1, 1564, he resigned the actual government of his dominions to his eldest son, Francis, though he retained his ducal title and certain prerogatives; and in December 1565 Francis was married to the Austrian archduchess Joanna (Joan), a diplomatic achievement celebrated with great festivity.

Finally on Aug. 27, 1569, Pope Pius V conferred the title of grand duke (granduca) of Tuscany on Cosimo. This title, however, was not recognized by the Habsburg powers or by the other Italian duchies. To gratify Pius, Cosimo in 1570 married Camilla Martelli, who had long been his mistress.