Surpassing 100-Gb/s data throughput is a key objective and an active area of research for sixth-generation (6G) wireless networks that can only be met by exploiting the terahertz (THz) frequency band (0.3–10 THz). THz channel modeling faces new challenges given the emerging relevance of scattering and molecular absorption in this frequency range as well as the lack of a reliable library of material properties. In this work, we address these challenges by measuring systematically the dielectric properties of 27 common building and office materials and reporting an in-house 3-D ray-launching (3D-RL) algorithm that uses the created material library and accounts for rough surface scattering and atmospheric attenuation. In order to validate the proposed algorithm, a channel sounder measurement campaign has been performed in a typical indoor environment at 300 GHz. Simulations and measurements show good agreement, demonstrating the need for modeling scattering and atmospheric absorption in the THz band. The proposed channel model approach enables scenarios at THz frequencies to be investigated by simulation, providing relevant knowledge for the development of ultrahigh-speed wireless communication systems.