Doping graphitic carbon nitride (g-C<inf>3</inf>N<inf>4</inf> (CN)), a visible-light-responsive photocatalyst, with rare earth elements has emerged as an effective strategy due to the structural and electronic improvements provided by the unique 4f electronic configuration of lanthanide ions. In this study, Tb-doped CN composites with different terbium (Tb) contents (0.25, 0.50, 0.75, and 1.0 mmol) were synthesized via a high-temperature calcination method using urea as the precursor. The structural, optical, and morphological modifications of the samples were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), ultraviolet–visible spectroscopy (UV–Vis), diffuse reflectance spectroscopy (DRS), scanning electron microscopy (SEM)/ field emission scanning electron microscopy (FESEM), inductively coupled plasma mass spectrometry (ICP-MS), fluorescence (FL), time-resolved photoluminescence (TRPL), Brunauer–Emmett–Teller (BET) surface area analyses and photocurrent measurements. Additionally, the results were theoretically supported by quantum chemical parameters obtained through density functional theory (DFT) calculations. The visible-light photocatalytic performance of the obtained materials was evaluated based on the degradation of methylene blue (MB) and tetracycline (TC), selected as representative pollutants from dye and antibiotic classes. Among the prepared samples, the catalyst 0.75 TbCN exhibited the most pronounced physicochemical modifications, including a significant increase in specific surface area (30.90 → 49.69 m² g⁻¹), suppressed FL intensity, and an extended carrier lifetime (8.12 → 9.52 ns). The photocurrent response increased by approximately 3.4 times, indicating markedly improved charge separation and transfer. DFT calculations further confirmed enhanced charge polarization and greater HOMO/LUMO delocalization. These synergistic effects resulted in substantially improved photocatalytic performance, with 3.77- and 1.94-fold higher removal efficiencies for MB and TC, respectively, compared to pure CN, demonstrating its potential as a promising photocatalyst candidate.