This paper emphasizes the structure, morphology, optical, and magnetic properties of sonochemically prepared terbium-substituted cobalt ferrite nanoparticles, CoTbxFe2-xO4 (0.00 <= x <= 0.10). The formation of cubic spinel nanosized ferrite structure was confirmed by X-ray diffraction (XRD), Field-emission scanning electron microscopy (FE-SEM), and Fourier-transform infrared (FT-IR) spectroscopy. The crystallites sizes were found in the range of 11-14 nm. Ultraviolet-visible percentage diffuse reflectance investigations were performed on pristine and Tb3+-doped cobalt spinet ferrite CoFe2O4 nanoparticles. The direct energy band gap (E-g) values were determined by applying the Kubelka-Munk theory and Tauc plots were found to be in a narrow band range of 1.37-1.44 eV. Analyses of magnetization versus the magnetic field (M(H)) were performed. The magnetic parameters, including the saturation magnetization (M-s), squareness ratio (SQR = M-r/M-s), magnetic moment (n(B)), remanence (M-r), and coercivity (H-c) were evaluated. The M(H) curves exhibited a soft ferrimagnetic nature. It was demonstrated that the Tb3+ substitutions strongly influenced the magnetization data. Indeed, the M-s, M-r, H-c, and n(B) values decreased with increasing Tb3+ substitution.