In this article, a wide stopband 20 dB harmonically suppressed low‐pass filter (LPF) using novel defected ground structures (DGSs) is proposed. The DGSs has been analyzed as a low pass filter which shows a significant harmonics suppression in the stopband. The lumped parameter equivalent of the DGSs has been developed to show its effectiveness. The modified equivalent circuit model of the filter helps in placing the transmission zero near −3 dB cutoff frequency. The LPF is designed on a 0.10 λg× 0.09 λg substrate size where λg is guided wavelength at −3 dB cut‐off frequency (fc) equal to1 GHz. The simulation shows a 20 dB harmonic suppression up to 50 fc. The prototype of the LPF has been developed and with the available vector network analyser, the S‐parameters have been measured upto 20 GHz (20 fc).The state of the art comparison of the LPF shows a high figure of merit equal to 26 250 which is higher than many recently published works.