The complexation behavior of lanthanide(III) ions with terephthalic acid (1,4-benzene-dicarboxylic acid) in 0.01 M KNO3 aqueous solutions was studied across a broad pH range and at two metal-to-ligand ratios using potentiometric titration combined with photoluminescence spectroscopy. Chemometric analysis of titration curves enabled the determination of relative molar fractions, stability constants, and probable stoichiometry of the formed complexes. In solutions with a 1:2 metal-to-ligand ratio, bis-complexes (two terephthalate ligands per lanthanide ion) predominated, while ligand-rich conditions favored the formation of tetra-complexes (four ligands per metal ion). In alkaline media, bis-complexes transform into mixed hydroxy-terephthalate species. Meanwhile, for the tetra-complexes, the addition of NaOH results in the formation of lanthanide ion hydroxo complexes without organic ligands. The structural diversity of these complexes, driven by the terephthalate ligand’s tendency to maximize denticity, suggested dimeric or oligomeric configurations. The stability constants and structural features of complexes in solution were found to align with those of known solid-state lanthanide–terephthalate polymers, highlighting their potential as models for polymeric structures.