原核生物界（Kingdom Monera）是1920年年代对生物分类的四界系统（动物界、植物界、原生生物界与原核生物界）所定义的一个界。1969年魏泰克(Robert Whittaker)提出的五界系统（增加了真菌界）仍然使用此概念。1970年代中期，明确了古菌与细菌的亲缘距离不比古菌与真核生物的距离更近，这导致了原核生物界这一概念被一部份人放弃。取而代之的是三域系统。

注意，原核生物界(Monera)在起初的一段时间里，并不包括蓝藻，而是因蓝藻具有光合作用而把它分入植物界。

Monera (Greek - μονήρης (monḗrēs), "single", "solitary") (/məˈnɪərə/ mə-NEER-ə) was a kingdom that contained unicellular organisms with a prokaryotic cell organization (having no nuclear membrane), such as bacteria. The taxon Monera was first proposed as a phylum by Ernst Haeckel in 1866. Subsequently, the phylum was elevated to the rank of kingdom in 1925 by Édouard Chatton. The last commonly accepted mega-classification with the taxon Monera was the five-kingdom classification system established by Robert Whittaker in 1969.