黄素单核苷酸（英语：Flavin mononucleotide，FMN）是一个由核黄素激酶自核黄素（维生素B₂）产生出来的生物分子，其功能包括了NADH脱氢酶在内的多种氧化还原酶的辅基并且作为生物感蓝光蛋白的辅因子。在催化性循环中，氧化型（FMN）、半醌型（FMNH与还原型（FMNH₂）这几种形式的可逆互变在多种氧化还原酶中发生。黄素单核苷酸是一个比烟酰胺腺嘌呤二核苷酸更强的氧化剂，并且因为它可以参与传递一个或两个电子，故其在此方面尤其有用。论到它在感蓝光蛋白中的作用，（氧化型）黄素单核苷酸从‘普通的’感光蛋白中“脱颖而出”作为信号状态而不是E/Z异构化。

Flavin mononucleotide (FMN), or riboflavin-5′-phosphate, is a biomolecule produced from riboflavin (vitamin B₂) by the enzyme riboflavin kinase and functions as prosthetic group of various oxidoreductases including NADH dehydrogenase as well as cofactor in biological blue-light photo receptors. During the catalytic cycle, a reversible interconversion of the oxidized (FMN), semiquinone (FMNH) and reduced (FMNH₂) forms occurs in the various oxidoreductases. FMN is a stronger oxidizing agent than NAD and is particularly useful because it can take part in both one- and two-electron transfers. In its role as blue-light photo receptor, (oxidized) FMN stands out from the 'conventional' photo receptors as the signaling state and not an E/Z isomerization.