A novel ultrawideband (UWB) bandpass filter (BPF) based on broadside coupled microstrip-to-coplanar waveguide (CPW) transition is proposed. The proposed structure consists of open circuited microstrip lines on the top arranged in back-to-back fashion and electromagnetically coupled to a short circuited CPW in the ground via the dielectric. This arrangement generates a smooth UWB having passband insertion loss <0.22 dB in simulation, and two transmission zeros at the lower and upper passband/stopband edges which enhances selectivity. To validate the theory a prototype is fabricated and its measured response tested against simulated data. The simulation and measured response are found to be in good agreement with each other. The proposed structure is quite compact in size measuring only 14.8 × 9.6 mm2. © 2016 Wiley Periodicals, Inc.