A photographing optical system includes eight lens elements which are, in order from an object side to an image side: a first lens element, a second lens element, a third lens element, a fourth lens element, a fifth lens element, a sixth lens element, a seventh lens element and an eighth lens element. The eight lens elements each have an object-side surface facing toward the object side and an image-side surface facing toward the image side. The first lens element has positive refractive power. The fifth lens element has positive refractive power. The object-side surface of the seventh lens element is convex in a paraxial region thereof. The image-side surface of the eighth lens element is concave in a paraxial region thereof. At least one lens surface of at least one lens element of the photographing optical system has at least one critical point in an off-axis region thereof.