姪子枢机（拉丁语：cardinalis nepos; 意大利语：cardinale nipote; 西班牙语：valido de su tío; 法语：prince de fortune)）是中世纪的一种枢机任命惯例。指以教宗姪子或外甥及类似亲戚关系而被任命为枢机者。由于教宗的神职人员身分不允许有后裔，因此姪子枢机可说是教宗所能找到的血源最相近的晚辈，也可说是他的亲信。因此在一段时期中许多重要职务都会由姪子枢机担当。事实上英文中的nepotism（裙带关系）就是从这种任人唯亲的行为中引申而来的。

A cardinal-nephew (Latin:cardinalis nepos; Italian:cardinale nipote; Spanish:valido de su tío; French:prince de fortune) is a cardinal elevated by a Pope who is that cardinal's uncle, or, more generally, his relative. The practice of creating cardinal-nephews originated in the Middle Ages, and reached its apex during the 16th and 17th centuries. 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.