Rogowski coil (RC) has been commonly employed in power electronics due to its benefits of galvanic isolation, high-current measurement capability, and simple integration. However, the bandwidth of RC is restricted to tens of megahertz because of the inextricable resonance issue caused by its self-inductance and parasitic capacitance. This limitation hinders the ability of the RC to measure fast current transients such as those of wide-bandgap (WBG) power devices. In this letter, the concept of transmission line RC (TL-RC) is proposed to address the limitations of inductance and capacitance. The design criteria of double-ended impedance matching are further derived to ensure optimal signal integrity of TL-RC at gigahertz levels. These two concepts jointly form the isolated current sensor design with a bandwidth exceeding 3 GHz. Comparative experiments confirm the ultrafast and isolated advantages of the TL-RC for the WBG applications.