Ignazio Busca (31 August 1731 in Milan – 12 August 1803 in Rome) was an Italian cardinal and Secretary of State of the Holy See. He was the last son of Lodovico Busca, marquess of Lomagna and Bianca Arconati Visconti. he took a degree in utroque iure in 1759 at the Università La Sapienza of Rome.
Created cardinal in the consistory of 30 March 1789, he received the Galero and the title of Santa Maria della Pace on 3 August 1789. He was appointed Secretary of State by Pope Pius VI in 1796. He participated in the conclave of 1800.