Sergius III (or IV) (died November 1073) was the duke of Amalfi from 1069, when he succeeded his father John II, until his death.
He and his father were expelled from Amalfi by his grandmother and uncle, Maria and Manso II, in April or May 1034.
In 1038, he returned with his father from exile in Naples, but they were forced out again the next year by Guaimar IV of Salerno.
In 1071, Sergius (now sole ruler) was at Montecassino for the reconsecration of that great abbey.
Gisulf was also present and Pope Alexander II worked out a peace between the two rulers.