Long-range synchronization of neural oscillations correlates with distinct behaviors, yet its causal role remains unproven. In mice, tests of anxiety-like avoidance behavior evoke increases in theta-frequency (~8 Hz) oscillatory synchrony between the ventral hippocampus (vHPC) and medial prefrontal cortex (mPFC). To test the causal role of this synchrony, we dynamically modulated vHPC-mPFC terminal activity using optogenetic stimulation. Oscillatory stimulation at 8 Hz maximally increased avoidance behavior compared to 2, 4 and 20 Hz. Moreover, avoidance behavior was selectively increased when 8 Hz stimulation was delivered in an oscillatory but not pulsatile manner. Furthermore, 8 Hz oscillatory stimulation enhanced vHPC-mPFC neurotransmission and entrained neural activity in the vHPC-mPFC network, resulting in increased synchrony between vHPC theta activity and mPFC spiking. These data suggest a privileged role for vHPC-mPFC theta-frequency communication in generating avoidance behavior, and provide direct evidence that synchronized oscillations play a role in facilitating neural transmission and behavior.