We have developed a simple and mechanically robust fiberoptic microsensor that enables optical detection of the sediment-water interface at a spatial resolution of <50 μm. The sensor measures with a tapered optical fiber the increased backscatter of near-infrared light near the sediment surface. To determine the sediment surface position independent of ambient light conditions, we developed a miniaturized opto-electronic system with an intensity-modulated laser diode (780 nm) as the light source and a photodiode as the detector. Laboratory tests of our system were done with artificial as well as with various natural sediments and biofilms. Fiberoptic microsensors for surface detection can be combined easily with both electrochemical and optical microsensors for oxygen or other reactive species.