A novel dual-mode planar wideband slotline antenna is proposed. A pair of short-circuited strips is symmetrically introduced along the slotline resonator near the nulls of electric field distribution of the second odd-order resonant mode. In this way, two resonant modes are excited in a single slotline radiator and are both used for radiation in a wide frequency range. With the help of these paired strips, the dominant half-wavelength mode can be gradually moved to the three halves-wavelength mode, resulting to achieve a wideband radiation with two resonances. Prototype antennas are then fabricated to experimentally validate the principle and design approach of the proposed planar slotline antenna. It is shown that the proposed slotline antenna’s impedance bandwidth could be effectively increased to 37.6% while keeping its inherent narrow slot configuration with length-to-width ratio (LWR) up to 42.00.