  We consider the scenario where dark matter (DM) is represented by an
ultra-light classical scalar field performing coherent periodic oscillations.
We point out that such DM perturbs the dynamics of binary systems either
through its gravitational field or via direct coupling to ordinary matter. This
perturbation gets resonantly amplified if the frequency of DM oscillations is
close to a (half-)integer multiple of the orbital frequency of the system and
leads to a secular variation of the orbital period. We suggest to use binary
pulsars as probes of this scenario and estimate their sensitivity. While the
current accuracy of observations is not yet sufficient to probe the purely
gravitational effect of DM, it already yields constraints on direct coupling
that are competitive with other bounds. The sensitivity will increase with the
upcoming radio observatories such as Square Kilometer Array.
