An imaging optical system includes, 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 and a fifth lens element. The first lens element with refractive power has an object-side surface being convex in a paraxial region and an image-side surface being concave in a paraxial region. The second lens element with positive refractive power has an image-side surface being convex in a paraxial region. The third lens element with negative refractive power has an image-side surface being concave in a paraxial region. The fourth lens element with positive refractive power has an image-side surface being convex in a paraxial region. The fifth lens element with negative refractive power has an image-side surface being concave in a paraxial region and having a convex shape in an off-axial region thereof.