An objective optical system is configured to be suitable to be implemented at a tip of an endoscope. The objective optical system is provided with a first lens unit having a first lens barrel and a first optical system including plurality of lens elements assembled in the first lens barrel, and a second lens unit having a second lens barrel and a second optical system including a plurality of lens elements. The first lens unit or the second lens unit includes an alignment lens which is movable in a direction perpendicular to the optical axis thereof. The alignment lens is a lens most sensitive with respect to an alignment error among the plurality of lenses included in the first optical system.