Water activity (aw) is a key physicochemical parameter governing moisture-driven processes in foods and dietary supplements and is widely used to characterize the availability of unbound water in complex matrices. Despite its relevance, market-based data on aw in commercial supplements are scarce, particularly for Kosovo. This study quantified aw in liquid, semi-solid, and solid dietary supplements and tested whether aw differs by dosage form and co-varies with moisture content and selected physicochemical parameters (pH, total dissolved solids (TDS), and electrical conductivity). Twenty products were collected from Kosovo retail markets and analyzed for water activity using a Rotronic HygroPalm HP23-AW-A meter (HC2-AW probe), for moisture content using Ohaus MB120/MB90 analyzers, and for additional physicochemical parameters using a calibrated multi-parameter meter. Liquid supplements exhibited higher aw values (0.509–0.951) than solid products (0.221–0.730), while the single semi-solid sample showed an intermediate value (0.616). Within the solid category, effervescent tablets consistently exhibited lower aw (0.221–0.320) than non-effervescent solids (0.419–0.730), indicating strong formulation-driven differences in unbound-water availability. The study identified statistically significant form-dependent differences in aw between liquid and solid supplements and demonstrated that, among solid formulations, effervescent matrices have lower aw than other solid forms. In solids, aw showed only a weak association with bulk moisture content, suggesting that matrix composition and processing can decouple aw from total moisture.