This study aimed to investigate the effect of the fatigue life enhancement of the rib-to-deck welded joints of orthotropic steel decks (OSDs) by extended peening treatment utilization. First, hammer peening was conducted around the weld bead of the test specimens of OSDs. It was found that the treatment on both the weld toes of the deck and the U-rib plates caused a deformation of the U-rib plate, i.e., peen forming. Then, fatigue tests were performed under R = 0.0, using an out-of-plane bending fatigue test machine by applying several magnitudes of pre-loadings, and the results showed that the specimens with peen forming had one JSSC class higher than the deck plate only. Finally, numerical simulations of peening treatment and peen forming were performed to reveal the reason for higher fatigue life enhancement by peen forming. Simulation results showed that peen forming would introduce about three times higher compressive residual stress at the weld root of the deck plate side than the peening treatment on the deck plate only, and induced compressive residual stresses around the weld root by peen forming were kept even after applying the pre-loadings. Therefore, it can be concluded that peen forming, as an extended peening treatment utilization, is highly effective in enhancing the fatigue life of OSDs.