Cells sense the biophysical properties of the surrounding microenvironment. In particular, the stiffness of the extracellular milieu can be interrogated by cells and integrated through mechanotransduction. Many cellular processes (like proliferation, migration, or differentiation) depend on the mechanical status of the cell (being largely dictated by actomyosin-dependent intracellular contractility), which in turn is influenced by the mechanical properties of the microenvironment. In this study, we explored the influence of substrate stiffness on the proteome of undifferentiated human umbilical cord matrix mesenchymal stem/stromal cells (hUCM-MSCs). The relative abundance of several identified proteins suffered significant changes when comparing between substrates. Interestingly, many of such proteins are related to the regulation of the actin cytoskeleton, a main player of mechanotransduction and cell physiology in response to mechanical cues.