Mongolia, Republic of, known from 1924 to 1991 as the Mongolian People's Republic, nation in Central Asia, bordered on the north by Russia and on the east, south, and west by China. The nation, sometimes called by its former name, Outer Mongolia, has a total area of 1,565,000 sq km (604,250 sq mi). The capital and largest city of Mongolia is Ulaanbaatar (Ulan Bator).