Segesterone acetate (SA) is a promising and recently approved drug substance used as a contraceptive. SA has two major polymorphic forms, Form I and II. We have shown through indirect analysis that Form I is the more thermodynamically stable polymorphic form at room temperature, however, during the manufacturing process of SA drug products the solid-state stability must be shown to be under control. In the present work, a systematic study has been done using X-ray powder diffraction (XRPD), Fourier Transformed Infrared spectroscopy (FT-IR), and room temperature Raman spectroscopy on both micronized and non-micronized SA powder samples. XRPD showed a crystalline structure in both powder samples with a distinct coexistence of the polymorphic Forms I and II which was confirmed by FT-IR and Raman spectroscopy. The study showed that after thermal annealing a noticeable reduction of the amount of polymorphic Form II was found in both samples. Our results suggest the possibility of reducing the amount of SA Form II by thermal treatment inducing an irreversible solid-state transition to yield the thermodynamically more stable polymorphic Form I. To quantify the ratio of polymorphs I and II we have implemented a method that can be used as a routine analysis step in the manufacturing process of SA.