We employed polypyrrole/graphene quantum dot (PPy/GQD) composites as a sensor for the simple and selective detection of catecholamine neurotransmitters (CNs), such as dopamine (DA), epinephrine (EP), norepinephrine (NE), which play vital roles in the peripheral and central nervous systems. The PPy/GQD composites showed strong fluorescence emission, which was significantly increased, by as much as greater than three times, compared to that of the pristine GQDs. In neutral solution, the CNs on the surface of the PPy/GQD composites were converted into a quinone structure, which triggered the fluorescence quenching of the PPy/GQD composites via a photo-induced electron transfer process. The CN concentration could be effectively monitored based on the quenching of the fluorescence signal of the PPy/GQDs. The quenching effect of DA is the fastest and most effective, followed by those of EP and NE, respectively, and the quenched fluorescence intensity of the PPy/GQDs was proportional to the concentration of DA (0.007-250 μM), EP (0.7-250 μM), and NE (5-500 μM). The present system was used for the quantification of CNs in human serum samples with acceptable results.