Nitrates are a supplement widely consumed by athletes, especially elite athletes, although until now, there has been no regulation of their labelling in terms of dosage, protocols, effects, etc. The aim of this study is to analyze the claims, dosages, protocols, formats and advertising aspects in the labelling of nitrate supplements, and to check that they follow the criteria of current evidence and international consensus. An observational cross-sectional study was conducted to analyze the content and the degree to which claims stated on the labelling or technical data sheets of nitrate supplements comply with those authorised by current European legislation and supported by existing scientific evidence. The products were searched for using Amazon and Google Shopping. In a total of 18 supplements, it was shown that only 27.8% showed one of the recognised, evidence-supported claims, and the number of nitrates on the label was in most cases within the recommendations (61.1%). ‘Improves performance’ and “Improves exercise efficiency” were the statements that were in line with the evidence. The remaining claims were inadequate or partially adequate and should have been removed or modified. A total of 55% of the sample had celebrities or influencers making claims and about 50% had an anti-doping seal. None of the external supplements were externally endorsed. This study shows that the supplements studied should modify their claims, as most of them do not comply with the recommendations of the main international organisations, and should present a recognised anti-doping seal to guarantee the absence of doping agents.