The study presents a new retrofit solution for strengthening metallic I-girders. The retrofit system involves two iron-based shape memory alloy (Fe-SMA, ‘memory-steel’) strips (each with a width and thickness of 50 and 1.5 mm, respectively) that are mechanically anchored (using friction clamps) to the girders. The shape memory effect (SME) of the Fe-SMA material has been used to activate/prestress the strips by heating to a predefined temperature. The main advantage of the proposed SMA-retrofit system is that, unlike conventional systems, it can prestress itself without a need for heavy hydraulic jacks, which then results in a significant reduction of the required time, labor works and cost of prestressing process. In order to evaluate the efficiency of the proposed retrofit system, in this study, a series of static and fatigue four-point bending tests were performed on a 6.4-m SMA-retrofitted beam. Five static tests were performed on a steel beam with different SMA prestressing levels and included a reference un-strengthened test. The test results indicated that the achieved prestressing levels (i.e., recovery stresses) in the Fe-SMAs for activation temperatures of 100, 160, and 260 °C were approximately 160, 330, and 430 MPa, respectively. The induced compressive stresses in the bottom flange were in the range of 10–30 MPa. It was demonstrated that the Fe-SMAs could be re-activated for multiple times even up to higher temperatures (than the initial activation temperature), which would then result in higher prestressing levels. These features make the proposed SMA-based system a versatile and adaptable retrofit solution. Furthermore, the SMA-strengthened beam with the maximum prestressing level (activation temperature of 260 °C) was subjected to 2 million load cycles with a load ratio of R = 0.2 and a loading frequency of 4.35 Hz. The results of the high-cycle fatigue (HCF) tests showed no slippage in the anchorage system and a stable prestressing in the Fe-SMA members during the tests, which demonstrates the reliability of the proposed system under HCF loading regime.