In this study, a low-molecular-weight polyethylenimine (2 kDa) was conjugated with dexamethasone to generate a PEI-Dexamethasone (PEI-Dexa) carrier for mRNA delivery. The conjugate was synthesized via thiol modification using Traut’s reagent followed by coupling with dexamethasone-21-mesylate. PEI-Dexa formed stable polyplexes with mRNA through electrostatic interactions, and complexation efficiency was evaluated at various weight ratios. In vitro transfection studies showed that PEI-Dexa/mRNA polyplexes achieved enhanced mRNA expression with significantly reduced cytotoxicity compared to unmodified PEI. Efficient transfection at low polymer-to-mRNA ratios indicated an improved therapeutic window and favorable biocompatibility. The self-assembly behavior of PEI-Dexa was examined using Nile red fluorescence to determine its critical aggregation concentration (CAC), suggesting micelle-like nanostructure formation that may enhance cellular uptake.