Numerous research studies are available in the literature on strengthening of steel members using carbon fiber reinforced polymer (CFRP) composites. Although it is clear that strengthening using prestressed CFRP plates has several advantages over non-prestressed reinforcements, less attention has been paid in the literature to develop theoretical formulations. Consequently, in this paper, unbonded and bonded prestressed CFRP-strengthened steel plates are analyzed and stresses in CFRP and steel substrate are analytically formulated. Furthermore, the effect of single-side retrofit, which is usually neglected in stress calculations, has been studied. Analytical formulations were then verified by the results of static tests on CFRP strengthened steel plates under tensile loading. The primary results show that prestressing could considerably reduce the tensile stresses in steel. Consequently, strengthening with prestressed CFRP laminates could be used as an efficient technique for strengthening of steel members, especially those prone to fatigue. However, the available capacity of the prestressed bonded reinforcements before debonding is much less than the developed mechanically anchored, prestressed unbonded reinforcements. Moreover, analytical modeling and experimental results of the current study show that neglecting the eccentricity in single-side CFRP-strengthened steel plates could result in an unsafe stress prediction in steel substrate.