A new design of wideband out-of-phase filtering power divider (FPD) with high selectivity is presented on the basis of a vertical microstrip-slotline-microstrip (M-S-M) transition in this paper. By wisely replacing the traditional quarter-wavelength open-circuited microstrip line in the M-S transition with an open/short stepped-impedance resonator, two transmission zeros (TZs) are generated, resulting in a controllable bandwidth and sharp skirt selectivity of the FPD. To well isolate the out-of-phase signals between two output ports, an isolation network consisting of grounded resistor and a microstrip line is located on the differential symmetrical plane. Equations for determination of TZs are derived from the equivalent schematic circuit. For demonstration, a prototype centered at 2.83 GHz with 3-dB fractional bandwidth of 64.8% is designed, fabricated and tested. Both simulated and measured results are displayed to verify the feasibility of our proposed design concept.