Photoluminescent composites consisting of a photoluminescent material dispersed in a suitable matrix have been applied in many applications, such as light-emitting diodes, solar concentrators, and anti-counterfeiting inks. The traditional method for the fabrication of composites by blending an as-synthesized photoluminescent material and a matrix is very challenging as it is difficult to obtain homogeneous composites. In this study, we have demonstrated a one-step method to prepare homogeneous composites by inducing the formation of in-situ photoluminescent centers in a stable matrix. Poly(methyl methacrylate) (PMMA) coated with o-phenylenediamine (oPD) was thermally annealed at 165oC for a duration of 5 minutes in an extruder to obtain green-emitting composites. The composites exhibited a broad absorption peak at 425 nm and an absorption shoulder at 495 nm. The emission spectrum of the composite was broad, ranging from 400 nm to 700 nm, and reached the maximum at 525 nm. The photoluminescent maximum position was independent of the excitation wavelength. The photoluminescent excitation spectrum of the composite resembled the absorption near 425 nm. Time-dependent density functional theory (TD-DFT) calculations suggested that 2,3-diaminophenazine and 3-amino-2-hydroxyphenazine are the main molecular fluorophores accounting for the optical properties of the composites. The synthetic method demonstrated in this study is transferable for preparing numerous photoluminescent thermoplastics.