Giulio d'Este (July 13, 1478 – March 24, 1561) was the illegitimate son of Ercole I d'Este, Duke of Ferrara, the result of an affair with Isabella Arduin, a lady in the service of Ercole's wife.
He is known for the conflicts he had with his half brother Ippolito d'Este, which culminated in a failed conspiracy that Giulio conducted against both Ippolito and another half brother Alfonso I d'Este, then Duke of Ferrara.