Introduction: Stiff-Person Syndrome (SPS) is a rare autoimmune neurological disorder characterized by progressive muscle rigidity and painful spasms, primarily affecting axial and proximal musculature. Its diagnosis can be challenging due to clinical overlap with other neurological conditions such as spasticity, dystonia, or functional movement disorders. The detection of antibodies against glutamic acid decarboxylase (anti-GAD) is a key biomarker that supports diagnosis. Methods: Two clinical cases of patients with manifestations consistent with classic SPS are described, and were evaluated in a specialized neurology service. Both patients underwent detailed clinical assessment, complementary studies, and serum testing for anti-GAD antibodies. Results: Both patients presented with progressive rigidity and fluctuating muscle spasms, predominantly involving axial musculature. After an extensive diagnostic workup, elevated anti-GAD antibody titers were documented in both cases, confirming the diagnosis of classic SPS. Treatment with medications enhancing GABAergic neurotransmission was associated with significant clinical improvement, evidenced by reduced rigidity and the decreased frequency of spasms. Conclusions: These cases highlight the importance of considering SPS in the differential diagnosis of progressive rigidity syndromes. Identification of anti-GAD antibodies is essential for diagnostic confirmation, and treatment that aims to enhance GABAergic neurotransmission can significantly improve symptoms and patient functionality.