The angle of view of an objective optical system is widened, while suppressing generation of lateral chromatic aberration. An objective optical system includes: a first lens group having a negative power an aperture stop and a second lens group having a positive power, in this order from an object side. The first lens group includes a negative single first lens, and a cemented lens formed by cementing a positive lens and a negative lens together, provided in this order from the object side. The second lens group includes a positive single fourth lens, and a cemented lens formed by cementing a positive lens and a negative lens together, provided in this order from the object side. The objective optical system satisfies Conditional Formula (1): 15.0<νd(RN)<18.6, wherein νd(RN) is the Abbes number of the negative lens in the cemented lens within the second lens group with respect to the d line.
