An ultrawideband (UWB) antenna covering the lower terahertz band is designed and numerically analyzed. A number of higher-order modes are excited and merged to provide the UWB response. The band-notch is obtained by applying graphene nanoribbons at the top of the radiating metallic patch. The antenna response can be set with the tunable single/dual/triple band-notch characteristics depending on the size and location of the applied graphene nanoribbons. The notched frequency band can be tuned by changing the chemical potential of the graphene nanoribbons by keeping the cutoff frequencies of the antenna response unchanged. Moreover, the antenna radiates like a monopole throughout the passband with consistent radiation pattern, high gain, and radiation efficiency.