Accurate far-infrared (FIR) radiative data are required for thermal radiation modelling, climate research, remote sensing, space instrumentation, and infrared optical metrology. In particular, directional hemispherical reflectance in the wavelength range from 25 µm to 100 µm is needed when both specular and diffuse radiation contributions must be considered. However, conventional integrating spheres cannot be directly transferred to this spectral range, because established diffuse coatings exhibit increased absorption and metallic surfaces tend to reflect predominantly specularly at long wavelengths. This creates a metrological gap for traceable far-infrared reflectance measurements with validated uncertainty budgets. In this work, a novel integrating sphere system for directional hemispherical reflectance measurements in the far-infrared spectral range was developed, realized, and experimentally validated. The system is based on a structured internal surface topology designed to provide diffuse-like angular redistribution while maintaining high reflectance. For this purpose, a Random Harmonic Function Surface topology (RHFS 800) was developed and optimized, transferred to a spherical geometry, manufactured by stereolithography-based additive manufacturing, and gold-coated. The integrating sphere was coupled to a Bruker VERTEX 80v Fourier-transform infrared (FTIR) spectrometer using a dedicated external optical relay system. The complete setup, referred to as the Far-Infrared Integrating Sphere (FIR-IS), enables controlled incident and detected radiation paths and supports directional hemispherical reflectance measurements using the substitution method. A radiometric correction model was implemented to account for finite sphere-wall reflectance, port geometry, and residual background signal. A spectral uncertainty budget was established in accordance with the Guide to the Expression of Uncertainty in Measurement (GUM), and expanded uncertainties were reported with a coverage factor of ?? = 2. The developed system was validated using samples with different radiative properties, including low-reflectance, high-reflectance, and semi-transparent coatings. The measured directional hemispherical reflectance spectra were compared with independent Physikalisch-Technische Bundesanstalt (PTB) reference data obtained from spectral emittance and directional reflectance measurements. The results show good agreement within the expanded uncertainty and demonstrate that the FIR-IS can reproduce both the overall reflectance level and characteristic spectral features of the investigated samples. This work establishes a design, fabrication, correction, and validation methodology for far-infrared integrating sphere radiometry. The FIR-IS provides a metrological tool for material characterization and supports applications in thermal radiometry, radiative transfer modelling, space instrumentation, and climate-related missions such as the European Space Agency (ESA) Far-infrared Outgoing Radiation Understanding and Monitoring (FORUM) mission.