In this paper a novel interdigital bandpass filter with under-coupled quarter-wavelength resonators is proposed. With this new structure, the spurious response up to the 3 harmonic passband can be suppressed significantly whilst the fundamental passband remains unchanged. Also, this design can take full advantage of the traditional interdigital filter design equations. In order to demonstrate the proposed filter performance, a 3 d-order Butterworth interdigital bandpass filter centered @900MHz with 15% fractional bandwidth was designed and implemented using both the proposed and the conventional structure. From the simulation and experimental results, the proposed filter obtains more than 10 dB suppression in average for the entire upper stopbandup to the triple of the midband-frequency value, and also 32 dB suppression of the 3rd harmonic spurious passband.