This paper investigates the hydraulics and fish guidance efficiency of a Horizontal Bar Rack-Bypass System (HBR-BS) installed at a hydropower plant with a design discharge of 33 m3/s. The HBR is placed at a horizontal rack angle of 38° to the flow direction with clear bar spacing of 20 mm. The BS has a vertical-axis flap gate with two openings. The HBR-BS complies with most literature design criteria. Velocity measurements were conducted using a moving-vessel Acoustic Doppler Current Profiler (ADCP). The fish monitoring study was conducted using a stow net, video, and ARIS sonar recordings. The fish monitoring calculations imply guidance efficiency of 84%, even for fish with total body lengths below 10 cm. Furthermore, the hydraulic results show that the flow field is favorable in terms of fish guidance due to a good alignment of the rack and the BS, corroborating the fish monitoring results. The results indicate that the HBR-BS functions not only as a physical barrier but also as a mechanical behavioral barrier for some small fish that are capable of physically passing the HBR. The present results are compared and discussed with the laboratory and field results from different studies in the literature.