This study investigated the effect of adding bioactive glass 45S5 (BG) of different particle sizes to dental composite on resin polymerization and shrinkage stress formation. Commercial flowable composite was mixed with either 15 wt% BG fillers (nanometric, micrometric, or hybrid BG) or inert barium glass. Real-time linear polymerization shrinkage and shrinkage stress were recorded, and the degree of conversion was measured using FTIR spectroscopy. The commercial (unmodified) composite developed significantly higher linear shrinkage and shrinkage stress than the groups with 15 wt% added inert or BG fillers. After adding inert barium glass, the composite showed significantly higher linear shrinkage than when micrometric BG was added. The addition of bioactive or inert glass fillers did not affect the degree of conversion. Shrinkage stress can be reduced by adding inert or bioactive fillers (nano- and/or microparticulate BG) without affecting monomer conversion.