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Transforming growth factor-β (TGF-β) signaling plays a central role in lung tissue homeostasis, coordinating epithelial repair, immune resolution, and stromal remodeling following injury. However, persistent or dysregulated TGF-β activation is a hallmark of both idiopathic pulmonary fibrosis (IPF) and lung cancer, two devastating pulmonary diseases that are traditionally studied as distinct entities. Emerging evidence suggests that this dichotomous view may obscure shared pathogenic mechanisms driven by aberrant TGF-β signaling dynamics. In this review, we synthesize experimental, translational, and clinical findings to propose a unifying framework in which IPF and lung cancer represent endpoints along a shared TGF-β-driven pathological continuum. We highlight how the duration and intensity of TGF-β signaling determine divergent cellular outcomes across epithelial cells, fibroblasts, and immune compartments-ranging from physiological wound repair to irreversible fibrotic remodeling and the establishment of a pro-tumorigenic niche. Particular emphasis is placed on the temporal transition from acute injury responses to chronic signaling states that promote epithelial plasticity, fibroblast fixation, immune suppression, and genomic instability. By integrating fibrosis and tumorigenesis into a single pathophysiological model, this review reframes TGF-β signaling as a time-dependent disease modifier rather than a disease-specific factor. This perspective provides a conceptual basis for therapeutic strategies targeting TGF-β signaling windows to intercept disease progression before irreversible fibrosis or malignant transformation occurs.