We offer an effective medium model for the elastic moduli of high‐porosity ocean‐bottom sediments. The elastic constants of the dry‐sediment frame depend on porosity, elastic moduli of the solid phase, and effective pressure. The model connects two end points in the elastic‐modulus‐porosity plane: the Hertz‐Mindlin modulus of a dense elastic sphere pack at critical porosity; and zero at 100% porosity. The elastic moduli of saturated sediment are calculated from those of the dry frame using Gassmann's equation. Unlike the suspension model, our model assigns non‐zero elastic constants to the dry‐sediment frame and can predict the shear‐wave velocity. Unlike various modifications of the travel‐time‐average equation, it is first‐principle‐based and contains only physical parameters. We justify this model by matching sonic data in shallow marine sediments and in an ODP well.