This article proposes a very compact planer open-loop bandpass filter (BPF) with asymmetric frequency response and covering the 2.5 to 2.6 GHz and 3.6 to 3.7 GHz spectrum for 4G and 5G applications, respectively. The microstrip BPF employs four open-loop ring resonators with 50 Ω tapped lines for input and output ports. To achieve sharper cut-off frequencies, one infinite and three finite transmission zeros are successfully generated on the upper and lower edges of the 4G and 5G passbands. The utilization of the planer four-section resonators not only reduces the size of the structure, but also provides either positive or negative cross-coupling. The cross-coupling coefficients between the resonators are optimized to resonate at the required frequency with proper bandwidth. The reported BPF is designed and optimized using CST software, and is implemented on a Rogers RO3010 substrate with a relative dielectric constant of 10.2 and a very compact size of 11×9×1.27 mm3. Good agreement is achieved between the simulated and measured results.