L-cysteine-capped Mn-doped ZnS quantum dots (QDs) were used for the determination of glutamic acid in foodstuffs. This method is based on measurement of the quenching of the phosphorescence intensity of the QDs after interacting with glutamic acid. A linear response was observed from 50 to 500 ng mL$^{-1}$ glutamic acid with a limit of detection of 6.79 ng mL$^{-1}$. Room temperature phosphorescence (RTP) intensity of the QDs was quenched rapidly upon the addition of the quencher and the reaction reached equilibrium within 2 min. The quenching mechanism of phosphorescence of Mn-doped ZnS QDs by glutamic acid is dynamic and the quenching constant was found as 1.9 $\times $ 10$^{5}$ M$^{-1}$. The developed method has some advantages such as freeness of interference from autofluorescence or common cations. The results showed that the proposed method is sensitive, selective, and fast, and does not require a derivatization step.