High strength metallic materials are known to experience significant degradation in mechanical properties thus load-bearing capacity in the presence of hydrogen (H), a phenomenon referred to as H embrittlement (HE). This phenomenon strongly threatens the safe operation of structural components used in hydrogen-containing environments, which retards the development of some of the pending infrastructures needed for a H economy. Alloys fabricated by additive manufacturing (AM) often possess a higher strength compared with their cast counterparts, due to the presence of high-density dislocations and the associated cellular substructures formed during rapid cooling and heating cycles. The high strength level increases the risk of HE of which the mechanisms and the mitigating strategies are unclear in additively manufactured materials. Here we show that an additively manufactured high entropy alloy (HEA) is highly susceptible to HE, due to H-enhanced decohesion of the high-angle grain boundary and the resulting intergranular cracking. We further enhance the HE resistance of such alloy by mixing ceramic nanoparticles during the AM process which poses a number of influences on the microstructure, strength and H tolerance. The enhanced resistance against H in the presence of ceramic nanoparticles can be attributed to the H trapping capacity stemming from particle itself and the associated microstructure changes induced by particles. The findings of this study provide valuable insights into the understanding of HE in additively manufactured materials and the microstructure design of additively manufactured HEAs with high strength and HE resistance.