Dietary inorganic nitrate has been linked to cardioprotective and ergogenic effects, yet public and professional knowledge of its properties remains poorly characterised. This study assessed dietary inorganic nitrate knowledge, beliefs, and purchasing behaviours among the general population and healthcare professionals internationally. This multinational cross-sectional survey recruited two parallel cohorts: a general population cohort via the Prolific platform (United Kingdom, United States, Australia; n = 2970) and a healthcare professional cohort recruited through professional email networks across 42 countries (n = 498). Cohort-specific Nitrate Knowledge Indices were developed (general population maximum 21 points; healthcare professional maximum 23 points), alongside a harmonised 20-point Cross-Cohort Comparable index spanning the seven knowledge domains shared by both instruments, enabling direct between-cohort and exploratory longitudinal comparison. Non-normally distributed scores are reported as median (interquartile range). Wilcoxon rank-sum and Kruskal-Wallis tests, Spearman correlations, and rank-biserial effect sizes were applied, with a two-sided threshold of p < 0.05. General population participants scored a median 8.9/21, and only 28.7% had heard of dietary inorganic nitrate, with significant between-country differences (United States highest; Kruskal-Wallis p < 0.001). Healthcare professionals scored a median 11.5/23, with marked regional variation (Oceania highest). On the comparable index, healthcare professionals outperformed the general population (median 10 vs 8; Wilcoxon p < 0.001), although both cohorts scored well below the maximum. Knowledge of the nitrate content of foods was the strongest correlate of overall performance (Spearman ρ = 0.91) yet showed no significant between-cohort difference (p = 0.070), representing a universal gap; bacon and sausage were misclassified as high-nitrate by 41.0% and 37.7% of participants respectively. Most participants (63.6%) wished to learn more about dietary nitrate, and 66-72% reported being likely or very likely to purchase a nitrate supplement when cardiovascular, cognitive, or metabolic benefits were described. In exploratory benchmarking, United Kingdom general population scores exceeded a 2024 reference cohort (p < 0.001; medium effect)(1), whereas United Kingdom healthcare professional scores were unchanged from a 2019 benchmark(2). Inorganic nitrate knowledge is limited across both public and professional groups internationally, with the ability to identify nitrate-rich foods a consistent shared gap regardless of professional background. Strong public interest and willingness to act on supporting evidence highlight an unmet need for coordinated public health communication and evidence-based professional training to close the gap between nitrate science and practice.