This article describes the development of a facile and efficient fluorescence sensor for the determination of glutathione (GSH). Presence of the antioxidant glutathione in blood serum is considered as a biomarker for catastrophe like colorectal cancer. Silver nanoclusters with strong fluorescence and good water solubility synthesized from relatively cheaper precursors are one of the species very much explored in fluorescence sensors and bioimaging. Here, Chicken egg derived-lysozyme functionalized silver nanoclusters (Lyz AgNCs) with red fluorescence emission has been synthesized and developed to a turn-off fluorescence sensor for GSH through which colorimetric determination is also possible. Due to the ground state ‘Ag–S’ interaction between Lyz AgNCs and GSH, the determination of the analyte is possible from 1.00 × 10−5 M to 1.00 × 10−6 M via fluorimetric and from 9.00 × 10−6 to 8.00 × 10−7 M via spectrophotometric techniques with a limit of detection 2.86 × 10−7 M and 4.76 × 10−7 M, respectively. Selectivity of the sensor has been studied and applicability of the sensor in artificial blood serum samples has been demonstrated.