The solid-state structure of N-methyl-serotonin {systematic name: [2-(5-hy-droxy-1H-indol-3-yl)eth-yl](meth-yl)aza-nium hydrogen oxalate}, C11H15N2O+·C2HO4 -, is reported. The structure possesses a singly protonated N-methylserotonin cation and one hydrogen oxalate anion in the asymmetric unit. In the crystal, the mol-ecules are linked by N-H⋯O and O-H⋯O hydrogen bonds into a three-dimensional network.