High-performance fibrous materials offer advantages such as high strength and low weight, making them promising for use in prestressed concrete. This work investigates the stress relaxation behavior of glass and carbon fiber reinforcements. Stress relaxation tests were conducted at load levels ranging from 10 % to 40 % of the materials’ tensile strength. A mathematical model was developed to describe this behavior, and the model parameters were determined. The stress relaxation of the fiber reinforcements was simulated and the resulting prestress loss was compared with that of traditional steel reinforcement. It was found that stress relaxation is more pronounced in glass and carbon fiber reinforcements than in steel. It was shown that the observed stress loss is primarily caused by the slippage of the fibrous reinforcement. Based on the findings, practical recommendations are provided for the application of these results in prestressed concrete design.