The object of research is the behavior of a prestressed composite beam with a cross-section composed of an I-shaped steel profile and concrete. The relevance of the research lies in the insufficient theoretical and experimental understanding of the flexural performance of such structures under prestressing conditions. In the examined configuration, the reinforcement is isolated from the main concrete section. Method. The research methodology includes full-scale laboratory testing, numerical simulation in ANSYS, and analytical calculations adapted to the specific features of the structure. Results. Experimental results were compared with calculation outcomes. The experimentally determined maximum bending moment was 111,499.7 N·m with a deflection of 5.91 mm; the deviations from the numerical model were 6.8 % in moment and 2.5 % in deflection, while the analytical model showed 2.9 % and 12.2 % deviation, respectively. The findings highlight the need to refine the stiffness reduction coefficient ks to improve the accuracy of deformation predictions.