Although we know much about the capacity of neurons to integrate synaptic inputs in vitro, less is known about synaptic integration in vivo. Here we address this issue by investigating the integration of inputs from the two eyes in mouse primary visual cortex. We find that binocular inputs to layer 2/3 pyramidal neurons are integrated sublinearly in an amplitude-dependent manner. Sublinear integration was greatest when binocular responses were largest, as occurs at the preferred orientation and binocular disparity, and highest contrast. Using voltage-clamp experiments and modeling, we show that sublinear integration occurs postsynaptically. The extent of sublinear integration cannot be accounted for solely by nonlinear integration of excitatory inputs, even when they are activated closely in space and time, but requires balanced recruitment of inhibition. Finally, we show that sublinear binocular integration acts as a divisive form of gain control, linearizing the output of binocular neurons and enhancing orientation selectivity.