Phytosterols have been receiving increasing attention due to their demonstrated health benefits. Micronization of phytosterol particles is desirable to enhance their physiological efficacy. Utilization of the environmentally friendly compressed fluid-based technology, called Depressurization of an Expanded Liquid Organic Solution (DELOS) was investigated to micronize a phytosterol mixture. A new polymorph of β-sitosterol, which was more crystalline than the native form, was obtained from the DELOS process regardless of the process conditions. In addition, particle size was reduced by an order of magnitude. The crystal structure of the new polymorph was determined from X-ray powder diffraction data. The proposed crystal structure for β-sitosterol, which contains a number of nearly isosteric vicariant molecules of lower molecular weight (mostly campesterol and campestanol, accounting in a crystalline solid-solution for nearly 10% of the molecular mixture) allows the presence of small cavities, in which some residual solvent molecules are temporarily trapped. Further structural analysis of the new and native polymorphs were performed by laser diffractometry, scanning electron microscopy, differential scanning calorimetry, thermogravimetric analysis, and X-ray powder diffraction. Findings of the study provide a route to obtain nutraceutical products that might show enhanced functional properties.