A parametric study was carried out to investigate the structural behaviour of composite steel–concrete T-beams under different prestressing conditions. The studied parameters include different cases of loading, tendon profiles, beam spans, initial prestressing levels and different dimensions of steel sections and concrete deck. The studied beams were modelled by the finite-element software Ansys. The effect of three geometrical parameters was investigated for three different tendon profiles. It was found that straight tendon profiles are more appropriate for beams under distributed loads, whereas a draped tendon profile is more convenient for beams under concentrated loads. In addition, the ratio of the tension flange area to the compression flange area is the most effective geometrical parameter on the ultimate resistance of the studied prestressed beams. For instance, increasing this ratio from 1 to 3 resulted in increasing the additional resistance of the beam due to prestressing from 15·4 to 46·1%. For composite beams, the presence of the concrete slab prevented the lateral–torsional buckling and accordingly minimised the effect of the span. Moreover, increasing the slab thickness-to-width ratio resulted in enhancing the average increase in beam strength from 13·5 to 19·9%.