Conventional casting induces gradual melting/solidification as the raw materials are melted and then solidified. On the other hand, repetitive and multiple melting/solidification cycles generated due to laser processing during additive manufacturing process cause appearance of residual thermal stresses in the material and of a heterogeneous microstructure with strongly anisotropic properties. Diffusion, especially grain boundary diffusion governs a number of fundamental phenomena in the additively manufactured (AM) materials, which include precipitation, phase transformation and creep. In the present study, we have investigated grain boundary diffusion in the AM CoCrFeMnNi high-entropy alloy (HEA). Three different scan strategies were used to produce the specimens with distinct orientations of the diffusion direction with respect to the microstructure features. The radioactive ⁶³Ni tracer has been applied. Grain boundary diffusion was found to be significantly enhanced in the AM HEA in comparison to the that in the well-annealed cast HEA. Kernel average misorientation maps have shown strain concentration at a fraction of grain boundaries. The relation between the anisotropic microstructure and the tracer penetration profiles is also discussed. For the first time, the measurements have discovered the existence of a specific, processing-induced non-equilibrium state of grain boundaries in the AM materials. This state was shown to be relaxed after a low-temperature heat treatment which did not induced any observable microstructure evolution. A similar grain boundary state has been reported previously for the materials produced by severe plastic deformation (SPD). The non-equilibrium states induced by the rapid melting/quenching and by the plastic deformation are comparatively discussed.