Joaquim António de Aguiar (Coimbra, 24 August 1792 – Lisbon, 26 May 1884) was a Portuguese politician. He held several relevant political posts during the Portuguese constitutional monarchy, namely as leader of the Cartists and later of the Partido Regenerador (English: Regenerator Party). He was three times prime minister of Portugal: between 1841 and 1842, in 1860 and finally from 1865 to 1868, when he entered a coalition with the Partido Progressista (English: Progressist Party), in what became known as the Governo de Fusão (English: Fusion Government).

He also served as minister of justice during the regency of Peter IV and in that capacity issued the 30 May 1834 law which extinguished "all convents, monasteries, colleges, hospices and any other houses of the regular religious orders". Their vast patrimony was taken over by the Portuguese State and incorporated into the Fazenda Nacional (the National Exchequer). This law and its anti-ecclesiastical spirit earned Joaquim António de Aguiar the nickname "O Mata-Frades" (English: "The Friar-Killer").