The transverse-optical Γ-point phonon mode was studied in the stability range of hcp Mg up to 50 GPa by Raman spectroscopy. A positive frequency shift is observed with increasing pressure in good agreement with recent theoretical studies using the pseudo-potential method. Unexpected broadening and splitting of this mode occurs under pressure. The separation increases to 20 cm^-1 at 50 GPa. Slight structural modifications, anharmonic effects or an unusual pressure-induced dispersion around the Γ-point, not observed in other hcp metals so far, are suggested as possible explanations for the splitting.