At room temperature, the contact of Tb3+ ions dissolved in bidistilled water with a polystyrene film incorporating 2-pyridone (PS-Pyr) exhibits terbium-sensitized photoluminescence (PL) when excited at 295nm. The Tb3+-sensitized emission is attributed to a complex formation as well as a highly efficient and selective energy transfer from 2-pyridone to Tb3+ ions. This has been utilized as the basis for a selective spectrofluorimetric detection of Tb3+. The association constant and the lifetime of the formed complex were determined; also, the sensing mechanism of the optosensor was proposed. The linear response range covers a concentration range of Tb3+ from 5.0×10−9 to 1.0×10−6molL−1. The LOD and LOQ were 2.9×10−9 and 9.7×10−9molL−1, respectively. The sensitizer exhibits a unique and a highly selective energy transfer towards Tb3+ ions over a large number of interfering ions. Moreover, the proposed optosensor was easy to prepare, has a relatively fast response time ∼30s and high stability for at least 42 days. The optosensor has been applied to the determination of terbium ion in synthetic samples with high sensitivity, good reproducibility and accuracy.