Ternary phosphate glass doped with different terbium concentration (up to 35 wt.%) is synthesized via standard melt-quenching technique. The effect of terbium content on physical, optical, and magneto-optical properties is investigated. Two glass transition regions on DSC curve at 35 wt.% Tb₂O₃ demonstrate phase-separation process. Transmittance spectra demonstrate maximum 0.2 cm⁻¹ level in optical transmittance window of 400–1600 nm limited by standard absorption bands for terbium-containing materials. Metaphosphate groups in as-synthesized glass structure are predominant over polyphosphate complexes with no ultraphosphate units regardless to composition changes. After the additional heat treatment the partial depolymerization of glass structure occurs resulting in formation of big amount of Q⁰ and Q¹ units. Glass refractive index varies from 1.5872 to 1.5814 with terbium content increase. The concentration dependence of Verdet constant is non-linear due to the presence of Tb⁴⁺ ions, which was confirmed by the XPS analysis results. The value of 15 rad/T/m at 1064 nm is reached for 35 wt.% Tb₂O₃-doped glass. The results indicate the viability of the considered glass series with high terbium oxide concentration to be attractive media for magnetooptical devices working in VIS and near IR ranges.