Using Brillouin scattering, we measured the single-crystal elastic constants (C ij's) of a prototypical metal-organic framework (MOF): zeolitic imidazolate framework (ZIF)-8 [Zn(2-methylimidazolate) 2], which adopts a zeolitic sodalite topology and exhibits large porosity. Its C ij's under ambient conditions are (in GPa) C 11=9.522(7), C 12=6.865(14), and C 44=0.967(4). Tensorial analysis of the C ij's reveals the complete picture of the anisotropic elasticity in cubic ZIF-8. We show that ZIF-8 has a remarkably low shear modulus G min1GPa, which is the lowest yet reported for a single-crystalline extended solid. Using ab initio calculations, we demonstrate that ZIF-8's C ij's can be reliably predicted, and its elastic deformation mechanism is linked to the pliant ZnN 4 tetrahedra. Our results shed new light on the role of elastic constants in establishing the structural stability of MOF materials and thus their suitability for practical applications.