A multiband monopole slot antenna includes a ground plane, a dielectric substrate, a radiating portion, and a microstrip feedline. The dielectric substrate is connected to an edge of the ground plane and extends toward the opposite direction of the ground plane. The radiating portion is on the metal surface of the dielectric substrate and includes a first monopole slot, a second monopole slot and a third monopole slot. The microstrip feedline is on the surface opposite to the metal surface of the dielectric substrate. A first end of the microstrip feedline is connected to a signal source, and a second end of the microstrip feedline is an open end. The microstrip feedline passes over the first, second, and third monopole slots. A section of the microstrip feedline which passes over the third monopole slot is parallel to the third monopole slot, and the microstrip feedline is generally of a step shape.