Hyacinthe Sigismond Gerdil, CRSP (23 June 1718 – 12 August 1802) was an Italian theologian, bishop and cardinal, who was a significant figure in the response of the papacy to the assault on the Catholic Church by the upheavals caused by the French Revolution.

He was appointed as a consultor to the Holy Office in 1776 by Pope Pius VI, moving to Rome, where he took a residence next to the General Motherhouse of the Barnabite Order at the Church of San Carlo ai Catinari.

The following June he again named a cardinal in petto by Pope Pius, receiving the red hat on 18 December 1777 and the titular church of San Giovanni a Porta Latina on 30 March 1778.

Gerdil was member of the committee of cardinals which examined and refined the text of the response against the Puntuazione di Ems, issued by the archbishops of Germany in which they defied papal authority.

From 1790 to 1794, he was member of the committee which prepared the final draft of the papal bull, Auctorem fidei (28 August 1794), which condemned the propositions of the Synod of Pistoia, a similar stand by the bishops of Tuscany.

Gerdil's numerous works written in Latin, Italian, and French on various subjects of dogmatic and moral theology, canon law, philosophy, pedagogy, history, physical and natural sciences, etc., form twenty volumes in quarto (ed.

Rousseau" (Turin, 1765), reprinted in a new edition under the title "Anti-Emile"; "Exposition des caractères de la vraie religion", written in Italian (translated into French, Paris, 1770), etc.