Abstract Terahertz time-domain spectroscopic polarimetry (THz-TDSP) method was used to experimental study polarization properties of unaligned single-wall carbon nanotube thin films with di˙erent geometric parameters on transparent float glass substrates in a frequency range from 0.2 THz to 0.8 THz (corresponding to a wavelength range from ∼1.50 mm to ∼0.37 mm) at a controlled room temperature of 291–293 K, and a relative humidity of 40–45 %. Frequency dependences of azimuth and ellipticity angles of a polarization ellipse (PE) of electromagnetic waves transmitted through the samples, and PEs at the frequencies of 0.2 THz, 0.5 THz, and 0.8 THz were obtained for values of 0.2 Wcm ⁻ 2, 0.6 Wcm ⁻ 2, and 1.0 Wcm ⁻ 2 of an external 980 nm near infrared optical pumping, with an external static magnetic field of ∼0.3 T. Polarization properties were calculated from temporal waveforms of signals transmitted through the samples at the parallel and the crossed by 45° positions to a transmission direction of the polarizers. A change of 15° in the azimuth angle, and of 10° in the ellipticity angle was achieved. The results show that by using carbon nanomaterials-based structures it is possible to devise efficient and affordable magneto-optically tunable polarization modulators that can be used in the advanced areas of terahertz nanoscience and nanotechnologies.