We report on the experimental observation of confinement-induced new phase appearance in acetylsalicylic acid (ASA)-porous glass (PG) composites. In this study, ASA was embedded in PG host matrices of various pore widths (15-200nm). The Raman spectra and positron annihilation lifetime measurements exhibit the existence of ASA nanocrystals in the PG matrix. The DSC data revealed that the melting temperature T-M and excess specific heat decrease with decreasing the size of embedded ASA nanocrystals. The close inspection of the T-M dependence versus diameter of filled pores has shown that the ASA crystallizes in polymorph II in confined matrix. Moreover, it was demonstrated that the ASA spatial confinement results in the appearance of new polymorphic phase in the investigated compositeshighly likely form ASA III. Both the changes in ASA melting temperatures due to the volume constraints and the decrease in specific heat may be crucial for the bioavailability of the drug.