This article presents an elaborate study on a compact wideband filtering out-of-phase power divider (PD) with a synthesis procedure based on a multilayer packaged stripline. The power splitting along with filtering performance is realized by a modified Marchand balun structure, where the length of the input feeding line is set as one wavelength. To achieve high port-to-port isolation, a half-wavelength line with a centrally shunted grounded resistor is connected between two out-of-phase output ports. By virtue of the stripline, a broadside coupled line is adopted to achieve the required coupling strength for the wideband performance. Based on the deductive equivalent circuits, synthesis design equations are derived, and design procedures are given to achieve a three-pole quasi-elliptic filtering response as well as wideband isolation. Finally, a circuit example implemented by multilayered printed circuit board (PCB) lamination is presented with simulation, fabrication, and measurement for verification. Results indicate that the realized filtering PD has a compact size and performs well in both filtering, power dividing/combining, and phase balance.