The effect of boron (B) addition on the microstructure and mechanical properties of a refractory high-entropy alloy (RHEA) was investigated. The Al0.1CrNbVMo RHEA was doped with 0.015 at.% of B by the powder metallurgical process, and its mechanical properties were compared with undoped RHEA. The microstructures of the undoped and doped RHEAs consisted of a single body-centered cubic matrix phase with homogeneously distributed Al2O3 inclusions. The doped B atoms segregated along grain boundaries and did not form any boride phases. Boron segregation at the grain boundaries was characterized by atom-probe tomography. Compared with the undoped RHEA, the B-doped RHEA showed improved yield strength of 2933 MPa (15% improvement) and ductility of 25.9% (26% improvement). Adding B transformed the fracture surface from intergranular to transgranular due to enhanced grain boundary cohesion induced by B segregation, which resulted in improved ductility. Quantitative analysis was conducted on the undoped and doped alloys to investigate the effect of B on yield strength. The improved yield strength resulting from B doping was attributed to the combined effects of Orowan, dislocation, and B interstitial strengthening. This work demonstrates that B doping of RHEAs is a promising way to overcome their limited room temperature ductility, while maintaining excellent strength.