An imaging optical lens assembly includes nine lens elements which are, in order from an object side to an image side along an optical path: 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, an eighth lens element and a ninth lens element. The first lens element has positive refractive power. The eighth lens element with positive refractive power has an image-side surface being convex in a paraxial region thereof. The ninth lens element has an image-side surface being concave in a paraxial region thereof, and the image-side surface of the ninth lens element has at least one convex critical point in an off-axis region thereof.