Samples of the magneto-active material—Tb3+:Y2O3 ceramics with Tb3+ ion concentrations of 10%, 20%, 30%, and 100% (Tb2O3)—were prepared and studied. The wavelength dependence of Verdet constant in the 380 nm–1750 nm range was approximated for all investigated ceramic samples and was predicted for a pure Tb2O3 material. Tb2O3 ceramics demonstrates a more than three times higher Verdet constant in comparison with terbium gallium garnet crystal or ceramics. The linear dependence of the Verdet constant on Tb3+ ion concentration in the Tb3+:Y2O3 ceramics was demonstrated. The obtained data will be useful for fabricating magneto-optical elements of Faraday devices based on Tb3+:Y2O3 with arbitrary Tb3+ ion concentration operating at room temperature in the wavelength range of 380 nm–1750 nm.