1866 in Argentina

Events in the year 1866 in Argentina.

Incumbents

 * President: Bartolomé Mitre
 * Vice President: Marcos Paz

Governors

 * Buenos Aires Province: Mariano Saavedra (until 1 May), Adolfo Alsina (starting 1 May)
 * Mendoza Province:
 * until 1 November: Carlos González
 * 1 November-11 November: Melitón Arroyo
 * starting 11 November: Carlos Juan Rodríguez
 * Santa Fe Province: Nicasio Oroño

Vice Governors

 * Buenos Aires Province: vacant

Events

 * January 31 – Paraguayan War: The Battle of Pehuajó takes place when about 1500 Paraguayan troops, commanded by General Francisco Isidoro Resquín and Lieutenant Celestino Prieto, engage in a surprise attack Argentinian and Uruguayan battalions with around 2000 men, led by General Emilio Conesa; the Paraguayans are successful and the joint Argentine-Uruguayan force suffers heavy casualties.
 * May 24 – Paraguayan War: First Battle of Tuyutí is an allied victory.
 * July 18 – Battle of Boquerón (1866)
 * September 1–3 – Battle of Curuzú
 * September 22 – Battle of Curupayty

Deaths

 * February 15 – Juan Gregorio de las Heras, military leader (born 1780)