This paper presents an ultra-wide stopband microstrip low-pass filter that utilizes elliptic resonators with a triple-coupling effect and a rectangular-shaped suppressor cell. The elliptic resonators with a long base length are responsible for creating a cutoff frequency at 1.11 GHz and a narrow stopband less than 5 GHz. A low coupling effect is achieved between two elliptic resonators by incorporating low-impedance stubs. To further enhance the coupling effect and increase the number of transmission zeros, three elliptic resonators with long base lengths are placed adjacent to each other, resulting in a triple-coupling effect. Additionally, the design includes two elliptic rectangular-shaped resonators to form an effective suppressor cell, which significantly expands the stopband bandwidth of the filter. The proposed filter is then analyzed using an LC equivalent circuit. Finally, the filter is fabricated on RO4003 substrate and measured. The measured insertion and return losses in the passband are 0.65 and 14.3, respectively. The filter exhibits a stopband bandwidth of 16.47 GHz, and its size is compact, equivalent to 0.0057 λg2. Through comparison with previous works and the obtained measurement results, it is evident that the proposed low-pass filter offers significant advantages in terms of ultra-wide stopband bandwidth and compact size.