The objective of this project was to design a monitoring system capable of detecting and quantifying tritium in-situ in ground and surface waters, and in water from effluent lines prior to discharge into public waterways. The design work was successfully completed with the predicted capability to detect tritium levels below 20 nanocuries per liter. The designed system was based on the detection of the low energy beta radiation from the radioactive decay of tritium using a special form of scintillating optical fiber directly in contact with the water to be measured. To support the design, laboratory tests were performed in several areas. Different types of scintillating fiber were tested to determine which would provide optimum system performance. The fibers contained a fluor material in a special cladding configuration which optimizes the absorption of beta radiation. The tritium detection system consists of an immersible sensor module containing the optical fiber and detection electronics as well as signal processing electronics. An umbilical cable is used to interconnect the components. The system design goals included optional permanent installation for routine water monitoring in wells, process and effluent lines or as a potential portable survey tool which could be moved from one location to another. Not all the design goals were met due to the large physical size of the immersible sensor module. Discussed in this paper are the design details of the in-situ tritium beta detector, the tests performed, and results obtained. The work was supported by U.S. Department of Energy (DOE) contract number DE-AC21-96MC33128.