Events in the year 1866 in Paraguay.
Incumbents President: Francisco Solano López Vice President: Domingo Francisco Sánchez
Events January 31 - Battle of Pehuajó May 2 - Paraguayan War: Battle of Estero Bellaco May 24 - Battle of Tuyutí July 18 - Battle of Boquerón (1866) September 1–3 - Battle of Curuzú September 22 - Battle of Curupayty Deaths July 18 - Elizardo Aquino, general, killed in action