Despite numerous studies on hydrogen embrittlement in conventional alloys and high-entropy alloys (HEAs), the researches on hydrogen embrittlement of eutectic HEAs are still extremely rare, while these alloys have shown exceptional strength-ductility synergy at ambient conditions. Recently, additive manufacturing (AM), as a revolutionary technique, has been used to deposit HEAs including eutectic ones. AM imparts both the complex geometry and the distinct microstructures (e.g. very fine lamellae at the submicron-level) to the as-deposited parts. In this study, the hydrogen embrittlement susceptibility of AlCoCrFeNi2.1 eutectic HEA in both as-cast state and AM state was investigated through in-situ uniaxial tensile test in a scanning electron microscope. Combining several advanced scanning electron microscopy-based characterization techniques including detailed fractography, electron backscattered diffraction (EBSD) and electron channeling contrast imaging (ECCI), the deformation and the hydrogen embrittlement behavior of the investigated eutectic HEA have been intensively discussed. The AM material is more susceptible to hydrogen embrittlement in comparison with the as-cast material. The finer microstructure and the higher density of boundaries in the AM material are responsible for the hydrogen uptake. Based on the EBSD analysis, normal high-angle grain boundaries are more susceptible to H-induced cracking than low-angle grain boundaries and phase boundaries.