Chemosensors provide an efficient and low-cost approach to the detection of various metal ions and small molecules. In the present work, we present a detailed investigation of a simple, easily-prepared, yet efficient, coumarin-based sensor for Hg(2+). The sensor is based on a fluorescent group of coumarin. When exposed to Hg(2+), the sensor solution shows a significant color change from yellow to red, and a fluorescence change from green to orange. Further study revealed that the sensor responds solely to Hg(2+), and shows excellent selectivity even in the presence of other potential competition metal ions, including Na(+), K(+), Co(2+), Fe(2+), Cd(2+), Cu(2+), Ni(2+), Ag(+), Zn(2+), Mg(2+), Mn(2+), Pb(2+), Fe(3+). Furthermore, the sensor has been successfully applied to detect Hg(2+) in a form of test paper, which may promote easy and effective application of the sensor.