An effective force field for hydrophobic interactions is developed based on a modified potential of-mean-force (PMF) expansion of the effective many-body interactions between nonpolar molecules in water. For the simplest nonpolar solutes in water, hard particles, the modified PMF expansion is exact in both limiting cases of infinite separation and perfect overlap. The hydrophobic interactions are parametrized by using the information-theory model of hydrophobic hydration. The interactions between nonpolar solutes are short-ranged and can be evaluated efficiently on a computer. The force field is compared with simulation data;for alkane conformational equilibria in water as well as a model for the formation of a hydrophobic core of a protein. The modified PMF expansion can be extended to solutes with attractive interactions, The observed accuracy, computational efficiency, and atomic detail of the model suggest that this simple hydrophobic force field can lead to a molecular alternative for phenomenological surface-area models with applications in ligand-binding and protein-folding studies.