A trench-structured silicon p-n-junction photodiode to detect penetrating hard X-ray photons with high detection efficiency at high speed has been proposed and fabricated. When compared with other compound-type semiconductors, silicon is inexpensive, nonhazardous, and offers excellent charge transport properties. Therefore, the long trench photodiode can enhance the detection efficiency for X-rays, and X-rays’ detection efficiency of more than 80% has been obtained by irradiating X-rays with a tube voltage of 80 kV from the horizontal direction of the sensor substrate along the length of the trench photodiode. In addition, silicon exhibits intrinsically excellent carrier mobility and long recombination lifetime, enabling rapid collection of signal charge generated by the photoelectric absorption of X-ray photons without loss. However, if the sensor length is increased to improve the detection efficiency, then the detector capacitance also increases, and this results in a slower response. In other words, a trade-off exists between the X-ray detection efficiency and the response time. In this study, a silicon trench photodiode with a length of 20 mm was fabricated, and a signal rise time of 12 ns was obtained under irradiation by 59.5 keV hard $\gamma $ (X)-ray photons. Process and device simulations performed using technology computer-aided design (TCAD) and circuit simulations using LTspice corroborated this rise time. Therefore, we have demonstrated that the proposed X-ray photon sensor exhibits short response times while maintaining high X-ray detection efficiency, as required for photon-counting X-ray computed tomography (CT).