An objective optical system includes, in order from an object side to an image side, a front group having a negative refractive power, an aperture stop, and a rear group having a positive refractive power. The front group includes a negative single lens, the rear group includes, in order from the object side, a positive single lens and one or more cemented lenses, and following conditional expressions (1), (2), (3), and (5) are satisfied. 1.95