A novel diamond-anvil cell (DAC) adaptation for far infrared (FIR) cryogenic magnetospectroscopy in a small-bore magnet is reported. A design featuring a multibellows ram is employed to generate amplified force for in situ pressure tuning of the DAC at cryogenic temperatures without increasing the overall diameter of the apparatus. A constructed version, using a 26-mm-diam double-bellows ram, is capable of producing 5 kN force (scalable to 10 kN with four bellows) when driven by 4He at 4.2 K. Signals are enhanced using paraboloidal cones to focus the FIR radiation on the sample and collect the transmitted signal. It is feasible to record transmission spectra to energies as low as 80 cm−1 with this apparatus using appropriate gasket-hole sizes and detectors. The pressure, magnetic field, and temperature can be tuned independently in the ranges 0–20 GPa (in steps as small as 0.05 GPa), 0–15 T, and 2–300 K. We present high pressure (4.2 K) Fourier-transform FIR spectra at fixed magnetic fields, and laser (118.8 μm) magnetospectroscopy data on the 1s–2p+ transition of Si donors in GaAs measured with this apparatus.