cosimo iii de' medici (14 august 1642 – 31 october 1723) was the penultimate grand duke of tuscany. he reigned from 1670 to 1723, and was the elder son of grand duke ferdinando ii of tuscany. cosimo's fifty long reign, the longest in tuscan history, was marked by a series of regulatory laws. his reign also saw tuscany's bad finances become even worse. he was succeeded by his elder surviving son, gian gastone de' medici, when he died, in 1723.