Photodynamic therapy (PDT) is known to be effective in the photorejuvenation of photoaged skin. However, the molecular mechanisms of rejuvenation by PDT remain elusive. In this study, we aimed to understand the molecular events occurring during the photorejuvenation after PDT in dermal fibroblasts in vitro. First, we found that PDT conditions resulted in an increased fibroblast proliferation and motility in vitro. Under this condition, cells had increased intracellular reactive oxygen species (ROS) production. Importantly, PDT induced a prolonged activation of extracellular signal-regulated kinase (ERK) with a corresponding increase in matrix metalloproteinase (MMP)-3 and collagen type Iα messenger RNA and protein. Moreover, inhibition of PDT-induced ERK activation significantly suppressed fibroblast proliferation and expression of MMP-3 and collagen type Iα following PDT. In addition, NAC (an antioxidant) inhibited PDT-induced fibroblast proliferation and ERK activation indicating that prolonged ERK activation and intracellular ROS contribute to the proliferation of fibroblasts and the dermal remodeling process for skin rejuvenation. We also identified increased collagen volume and decreased elastotic materials that are used as markers of photoaging in human skin samples using histochemical studies. Results from this study suggest that intracellular ROS stimulated by PDT in dermal fibroblasts lead to prolonged activation of ERK and, eventually, fibroblast proliferation and activation. Our data thus reveal a molecular mechanism underlying the skin rejuvenation effect of PDT.