Fibrosis is characterized by persistent deposition of extracellular matrix (ECM) by fibroblasts. Fibroblast mechanosensing of a stiffened ECM is hypothesized to drive the fibrotic program; however, the spatial distribution of ECM mechanics and their derangements in progressive fibrosis are poorly characterized. Importantly, fibrosis presents with significant histopathological heterogeneity at the microscale. Here, we report that fibroblastic foci (FF), the regions of active fibrogenesis in idiopathic pulmonary fibrosis (IPF), are surprisingly of similar modulus as normal lung parenchyma and are nonlinearly elastic. In vitro, provisional ECMs with mechanical properties similar to those of FF activate both normal and IPF patient–derived fibroblasts, whereas type I collagen ECMs with similar mechanical properties do not. This is mediated, in part, by α v β 3 integrin engagement and is augmented by loss of expression of Thy-1, which regulates α v β 3 integrin avidity for ECM. Thy-1 loss potentiates cell contractility-driven strain stiffening of provisional ECM in vitro and causes elevated α v β 3 integrin activation, increased fibrosis, and greater mortality following fibrotic lung injury in vivo. These data suggest a central role for α v β 3 integrin and provisional ECM in overriding mechanical cues that normally impose quiescent phenotypes, driving progressive fibrosis through physical stiffening of the fibrotic niche.