The in-situ synthesis and photophysical properties of a p-hydroxybenzoic acid–terbium (PHBA–Tb³⁺) complex in sol–gel derived host materials have been investigated. The Raman scattering spectrum of a 1% TbCl₃–3% PHBA co-doped gel was the same as that of the PHBA–Tb³⁺ complex powders synthesized in ethanolic solution, indicating that the PHBA–Tb³⁺ complex was synthesized in the 1% TbCl₃–3% PHBA co-doped gel. The steady-state excitation and emission spectra of the 1% TbCl₃–3% PHBA co-doped gel, which were similar to those of the PHBA–Tb³⁺ complex powders, also indicated the synthesis of the PHBA–Tb³⁺ complex in the 1% TbCl₃–3% PHBA co-doped gel. The time-resolved spectra of the 1%TbCl₃–3% PHBA co-doped gel indicated an efficient ligand-to-metal energy transfer, which also confirmed the synthesis of the PHBA–Tb³⁺ complex in the gel. The luminescence decay curve of the 1% TbCl₃–3% PHBA co-doped gel could be fitted by a double-exponential function. Due to the less efficient non-radiative ⁵D₄ relaxation processes in the gel, the presence of the PHBA–Tb³⁺ complex in the gel led to greatly increased ⁵D₄ lifetimes.