An objective optical system includes: a negative first lens group; an aperture stop; and a positive second lens group, provided in this order from an object side. The first lens group includes a first lens, which is a negative single 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 fourth lens, which is a positive single 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): f1/f