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 $ u_{mu} rightarrow u_{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 $0.05~eV^2leq Delta m^2_{41}leq 0.5~eV^2$, the range of mass splittings that produce no significant oscillations over the Near Detector baseline.
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