A microstrip balanced-to-balanced filtering power divider (FPD) design with impedance transformation via a coupled three-line balun structure is proposed. The even/odd mode-analysis method is used to construct a three-port balun bandpass filter (BPF) with stub-loaded resonators and a three-line output structure after analysis and design to prescribe the filtering response of a proposed balanced-to-balanced FPD. To realize good isolation between the differential output ports, a complex impedance-isolation network is loaded by means of coupled lines in the input-feeding section. Theoretical formulas characterized by the coupling factors are provided with a specified source and load impedance ratio under the condition of perfect impedance matching at the centre frequency. According to the calculated S-parameters, design rules are summarized to determine the overall circuit size according to desired bandwidth and return loss. For validation, two balanced-to-balanced FPDs centred at 2.4 GHz with 50-to-50 Ω matching and 50-to-100 Ω transformation are designed, fabricated and tested. Good agreement between the simulated and tested results of the designed balanced-to-balanced impedance-transforming FPDs indicates not only high common-mode rejection but also good port-to-port isolation and good matching at all ports.