The effect of the addition of niobium oxide $({\rm NbO}_{2})$ on the critical current density of large, single grain bulk YBCO superconductors fabricated by top seeded melt growth (TSMG) has been investigated. The high field critical current density, $J_{c}$, of ${\rm NbO}_{2}$ doped YBCO is improved significantly even though $J_{c}$ in the low field region is generally lower than in undoped samples. Under these conditions, when field is applied parallel to the $c$ -axis $(H//c)$ , $J_{c}$ is enhanced by more than an order of magnitude at 4 T at 77 K to $2\times 10^{3}\ {\rm A/cm}^{2}$ for the ${\rm NbO}_{2}$ containing sample. This enhancement in $J_{c}$ is accompanied by a large increase in irreversibility field, $H_{\rm irr}$, from 4 T for the undoped sample to over 6 T. Enhanced flux pinning at high field is due to the formation of a nano-scale Nb-rich phase during high temperature sintering, melting and solidification of the bulk superconductor.