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 6.06$\times^{20}$ protons-on-target in the NuMI beam at Fermilab, we observe 95 neutral-current candidates at the Far Detector compared with events predicted assuming mixing only occurs between active neutrino species. No evidence for transitions is found. Interpreting these results within a 3+1 model, we place constraints on the mixing angles and at the 90% C.L. for , the range of mass splittings that produce no significant oscillations over the Near Detector baseline.