August Alexander Järnefelt (2 April 1833 – 15 April 1896) was a Finnish general, topographist, governor and senator.
Aleksander Järnefelt was born in Tohmajärvi, the son of crown overseer Gustav Adolf Järnefelt and Aurora Fredrika Molander. Aleksander married Elisabeth Clodt von Jürgensburg on 22 December 1857 in Saint Petersburg. Their children were Kasper, Arvid, Erik, Ellida, Ellen, Armas, Aino, Hilja and Sigrid. Armas, Arvid and Eero were famous Finnish cultural figures. Aino Järnefelt was married to composer Jean Sibelius.
After seven year's study in Hamina Cadet School, Järnefelt embarked on a career as an artillery officer in the Russian Army. After working under Wilhelm von Struve at Pulkovo Observatory, Järnefelt was commanded to topographic works in his native Finland which formed his main occupation for decades. By 1870, he was the head if the Russian topographic corps in Finland.
During and after the Turkish war (1877–1879), Järnefelt lead the topographic survey of many areas which Russia conquered from Turkey. These works, on which he also wrote a German-language scientific treatise Die astronomischen, geodätischen und topographischen Arbeiten auf der Balkanhalbinsel in der Jahren 1877, 1878 und 1879, earned him a promotion to general major.
Politically, Järnefelt was a fennoman of the Old Finnish Party. The party strove for the equal rights of Finnish language with the Swedish while stressing the importance of loyalty to the Emperor. Järnefelt took these aims seriously, sending his children to Finnish-speaking schools which was extraordinary for a nobleman of his time.