For decades, silicon (Si) has been widely used for the mass production of microelectronic circuits. Recently, as the thickness has been reduced to the nanometer scale, its application has expanded to various fields, including flexible and transparent 2D semiconductors. For the reliable and reproducible operation of such large flexible and transparent devices, obtaining precise information about the mechanical properties of low dimensional Si is crucial. Here, we demonstrate that a 2 nm-thick Si nanomembrane (NM) exhibits an extremely low Young's modulus of 3.25 GPa, a two-order smaller value than that of the bulk counterpart. Our systematic measurement of thickness-controlled Si NMs reveals the existence of significant size effect: The effective modulus rapidly changes from 180 GPa to 3.25 GPa under 25 nm to 2 nm thickness reduction. Our theoretical modeling successfully provides physical insight into the unique stiff-to-soft transition and extremely low modulus. We further demonstrate that the modulus of Si NMs can be tailored precisely via the control of surface morphology of membrane. This work therefore provides a comprehensive picture of how and why originally hard & stiff Si deforms so softly in the ultrathin 2D geometry, and proposes a new strategy to design the mechanical properties at nanoscale dimensions.