sixth grand duke of Tuscany, who reigned for 53 years (1670–1723), longer than any other Medici, but under whom Tuscany's power declined drastically.

Though Cosimo III traveled widely and spent money generously (in particular for the benefit of the church), he had a reserved manner aggravated by conjugal, political, and dynastic disillusionments, and he took refuge more and more in devout practices. After continually trying to represent her marriage as invalid, his frivolous consort, Marguerite-Louise d'Orléans, finally abandoned him in 1675 and went back to France. The humiliations that he had to endure in order to keep Tuscany neutral in the European conflicts of the age of Louis XIV were barely compensated by his navy's share in Francesco Morosini's victories over the Turks (1684).

When his brother and his two sons proved childless, Cosimo and his brilliant minister Carlo Rinuccini had to envisage the extinction of his dynasty and even proposed, at one point, that a republican form of government should be restored in Tuscany. The European powers, however, coveted his succession and ignored his remonstrances; and by the Treaty of London (1718) the Quadruple alliance—namely, the alliance of Great Britain, France, the United Provinces of the Netherlands, and the Holy Roman emperor Charles VI—decided that, on the extinction of the male line of the Medici, the grand duchy, together with Parma and Piacenza, should pass to Don Carlos de Borbón (the future Charles III of Spain). When Cosimo III's son and successor, Gian Gastone, died in 1737, the grand duchy of Tuscany in fact, however, passed to the dukes of Lorraine.