| Summary: | While the demand for <i>Spirulina</i> dietary supplements continues to grow, product inspection in terms of authenticity and safety remains limited. This study used the stable isotope ratios of light elements (C, N, S, H, and O) and the elemental composition to characterize <i>Spirulina</i> dietary supplements available on the Slovenian market. Forty-six samples were labelled as originating from the EU (1), non-EU (6), Hawaii (2), Italy (2), Japan (1), Portugal (2), Taiwan (3), India (4), and China (16), and nine products were without a declared origin. Stable isotope ratio median values were –23.9‰ (–26.0 to –21.8‰) for <i>δ</i><sup>13</sup>C, 4.80‰ (1.30–8.02‰) for <i>δ</i><sup>15</sup>N, 11.0‰ (6.79–12.7‰) for <i>δ</i><sup>34</sup>S, –173‰ (– 190 to –158‰) for <i>δ</i><sup>2</sup>H, and 17.2‰ (15.8–18.8‰) for <i>δ</i><sup>18</sup>O. Multivariate statistical analyses achieved a reliable differentiation of Hawaiian, Italian, and Portuguese (100%) samples and a good separation of Chinese samples, while the separation of Indian and Taiwanese samples was less successful, but still notable. The study showed that differences in isotopic and elemental composition are indicative of sample origins, cultivation and processing methods, and environmental conditions such that, when combined, they provide a promising tool for determining the authenticity of <i>Spirulina</i> products.
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