An optical imaging system includes seven 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 and a seventh lens element. Each of the seven lens elements has an object-side surface facing toward the object side and an image-side surface facing toward the image side. The seventh lens element has negative refractive power, and the object-side surface of the seventh lens element is concave in a paraxial region thereof. At least one of the object-side surface and the image-side surface of at least one lens element of the optical imaging system has at least one inflection point in an off-axis region thereof.