A new design of wideband out-of-phase filtering power divider (FPD) with an ultra-wide isolation band is presented in this paper. By utilizing a vertical microstrip-slotline-microstrip (M-S-M) transition, wideband filtering performance is achieved. With a special feeding structure at input port as well as two shunted open-ended stubs at the output ports, four transmission zeros (TZs) are generated, resulting in the 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. In addition, owing to the shunted stubs at output ports, two TZs are also generated in S, making the isolation bandwidth enlarged. For demonstration, generalized equivalent circuit is derived and analyzed. Finally, a prototype centered at 2.8 GHz with a 71 % of 15-dB return loss bandwidth is designed. Results indicate that the 15-dB isolation bandwidth of proposed out-of-phase FPD is up to 109 %.