A novel wideband 3-dB power divider is proposed and designed under triple-mode resonance of a cross-shaped microstrip-line resonator. A 50-25Ω impedance transformer is firstly formed by directly tap-feeding this triple-mode resonator at its two different positions. The bandwidth of this impedance transformer can be properly controlled by changing the characteristic impedance of the two loaded stubs. Afterwards, a 3-dB power divider is constructed using this 50-25Ω impedance transformer as its two arms in parallel. The design parameters of the proposed power divider are analyzed and initially determined based on its equivalent ideal transmission-line model. Finally, a 3-dB power divider is designed and fabricated. Simulated results are well confirmed by experiment, showing good wideband power splitting and port-to-port isolation performance.