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Petal size is a key indicator of the ornamental value of plants, such as <i>Petunia hybrida</i> L., which is a popular ornamental species worldwide. Our previous study identified a flower-specific expression pattern of a DNA-binding one finger (Dof)-type transcription factor (TF) <i>PhDof28</i>, in the semi-flowering and full-flowering stages of petunia. In this study, subcellular localization and activation assays showed that <i>PhDof28</i> was localized in the cell nucleus and could undergo in vitro self-activation. The expression levels of <i>PhDof28</i> tended to be significantly up-regulated at the top parts of petals during petunia flower opening. Transgenic petunia ‘W115’ and tobacco plants overexpressing <i>PhDof28</i> showed similar larger petal phenotypes. The cell sizes at the middle and top parts of transgenic petunia petals were significantly increased, along with higher levels of endogenous indole-3-acetic acid (IAA) hormone. Interestingly, the expression levels of two TFs, <i>PhNAC100</i> and <i>PhBPEp</i>, which were reported as negative regulators for flower development, were dramatically increased, while the accumulation of jasmonic acid (JA), which induces <i>PhBPEp</i> expression, was also significantly enhanced in the transgenic petals. These results indicated that <i>PhDof28</i> overexpression could increase petal size by enhancing the synthesis of endogenous IAA in petunias. Moreover, a JA-related feedback regulation mechanism was potentially activated to prevent overgrowth of petals in transgenic plants. This study will not only enhance our knowledge of the Dof TF family, but also provide crucial genetic resources for future improvements of plant ornamental traits.