Fluorescence sensors for trivalent europium ions (Eu3+) are seldom reported. We study the synthesis of water-soluble quaternary quantum dots (QDs) and investigate their fluorescence sensor application for detecting Eu3+ The as-synthesized glutathione (GSH)-capped AgZnInS (AZIS) QDs show great sensitivity and selectivity to Eu3+among 12 different metal cations. Detailed experimental results indicate that the fluorescence response of the AZIS QDs to increasing concentration of Eu3+ ([Eu3+]) include intensity quenching and peak wavelength blueshift. With the addition of OH-, the fluorescence response reverses. Electron transfer is considered to be the mechanism for the fluorescence quenching and peak wavelength blueshift of the GSH-capped AZIS QDs. Our work provides a new, to the best of our knowledge, method for the detection of Eu3+.