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[Display omitted] • A combination of high strength and ductility in TiZrNbTa HEAs was achieved by doping nitrogen. • Interstitial strengthening contributes to the enhanced strength. The improved ductility originates from the promoted planar dislocation slip inside grains. • Annealing further increases the strength and ductility of the nitrogen-doped TiZrNbTa HEA. • The improved strength and ductility in the annealed HEAs attributes to the nitrogen solid solution strengthening, in situ grain refinement and promoted dislocation cross slip. Refractory high-entropy alloys (HEAs) usually exhibit high strength but limited ductility. Here, we report high strength (∼1334 MPa) and large ductility (∼18.8%) in nitrogen-doped TiZrNbTa HEAs. The microstructural evolution and the corresponding mechanical properties of the as-cast/annealed nitrogen-doped HEAs were investigated. All as-cast (TiZrNbTa) 100- x N x (x = 0, 0.3, 0.6) HEAs exhibit a single-phase body-centred cubic structure. Interstitial strengthening contributes to the enhanced strength. The improved ductility of the 0.6 at.% nitrogen-doped HEA originates from the promoted planar dislocation slip inside grains. Spinodal decomposition and grain boundary (Zr, N)-enriched precipitates emerge when the nitrogen content is 0.9 at.%, which results in significant embrittlement. Isothermal annealing of the 0.6 at.% nitrogen-doped HEA at 800 °C to 1200 °C was conducted to further improve the mechanical performance. The complicated precipitates and grain boundary impurity segregation in the alloys annealed at 800 °C and 1200 °C lead to deteriorated strength and ductility. However, in the alloy annealed at 1000 °C, a slight increase in both strength and ductility appears due to solid solution strengthening, in situ grain refinement and promoted dislocation cross slip, which is induced by the coherent boundaries. This work provides a promising conduit to approach high strength and ductility in HEAs. [ABSTRACT FROM AUTHOR]