A cardinal-nephew (Latin: cardinalis nepos; Italian: cardinale nipote; Spanish: valido de su tío; Portuguese: cardeal-sobrinho; French: prince de fortune) was a cardinal elevated by a pope who was that cardinal's relative. The practice of creating cardinal-nephews originated in the Middle Ages, and reached its apex during the 16th and 17th centuries. The last cardinal-nephew was named in 1689 and the practice was abolished in 1692. The word nepotism originally referred specifically to this practice, when it appeared in the English language about 1669. From the middle of the Avignon Papacy (1309–1377) until Pope Innocent XII's anti-nepotism bull (a papal charter), Romanum decet pontificem (1692), a pope without a cardinal-nephew was the exception to the rule. Every Renaissance pope who created cardinals appointed a relative to the College of Cardinals, and the nephew was the most common choice, although one of Alexander VI's creations was his own son.

The institution of the cardinal-nephew evolved over seven centuries, tracking developments in the history of the papacy and the styles of individual popes. From 1566 until 1692, a cardinal-nephew held the curial office of the Superintendent of the Ecclesiastical State, known as the Cardinal Nephew, and thus the terms are sometimes used interchangeably. The curial office of the Cardinal Nephew as well as the institution of the cardinal-nephew declined as the power of the Cardinal Secretary of State increased and the temporal power of popes decreased in the 17th and 18th centuries.

The list of cardinal-nephews includes at least fifteen, and possibly as many as nineteen popes (Gregory IX, Alexander IV, Adrian V, Gregory XI, Boniface IX, Innocent VII, Eugene IV, Paul II, Alexander VI, Pius III, Julius II, Leo X, Clement VII, Benedict XIII, and Pius VII; perhaps also John XIX and Benedict IX, if they were really promoted cardinals; as well as Innocent III and Benedict XII, if in fact they were related to their elevators); one antipope (John XXIII); and two or three saints (Charles Borromeo, Guarinus of Palestrina, and perhaps Anselm of Lucca, if he was really a cardinal).