| Joaquim António de Aguiar | |
| Order: | Prime Minister of Portugal |
| Term Start2: | 9 June 1841 |
| Term End2: | 7 February 1842 |
| Monarch2: | Maria II and Fernando II |
| Predecessor2: | Count of Bonfim |
| Successor2: | Marquis of Palmela |
| Term Start1: | 1 May 1860 |
| Term End1: | 4 July 1860 |
| Monarch1: | Pedro V |
| Predecessor1: | Duke of Terceira |
| Successor1: | Duke of Loulé |
| Term Start: | 4 September 1865 |
| Term End: | 4 January 1868 |
| Predecessor: | Marquis of Sá da Bandeira |
| Successor: | Duke of Ávila |
| Birth Date: | 1792 8, df=yes |
| Birth Place: | Coimbra, Kingdom of Portugal |
| Death Place: | Lisbon, Kingdom of Portugal |
| Party: | Regenerator Party Progressist |
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 (en|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: Progressive Party), in what became known as the (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").