  We report results from the first search for sterile neutrinos mixing with
active neutrinos through a reduction in the rate of neutral-current
interactions over a baseline of 810\,km between the NOvA detectors. Analyzing a
14-kton detector equivalent exposure of 6.05$\times$10$^{20}$ protons-on-target
in the NuMI beam at Fermilab, we observe 95 neutral-current candidates at the
Far Detector compared with $83.5 \pm 9.7 \mbox{(stat.)} \pm 9.4 \mbox{(syst.)}$
events predicted assuming mixing only occurs between active neutrino species.
No evidence for $\nu_{\mu} \rightarrow \nu_{s}$ transitions is found.
Interpreting these results within a 3+1 model, we place constraints on the
mixing angles $\theta_{24}<20.8^{\circ}$ and $\theta_{34}<31.2^{\circ}$ at the
90% C.L. for $0.05~eV^2\leq \Delta m^2_{41}\leq 0.5~eV^2$, the range of mass
splittings that produce no significant oscillations over the Near Detector
baseline.
