Polycrystalline materials are widely used in engineering and material science applications, e.g. automobile, aerospace or renewable energy. The actual microstructure of the material at meso- and microscopic scales has a tremendous effect on the mechanical properties of the material and especially on the fracture mechanisms. Since fracture is the major failure mechanism in most construction materials, it is vital to analyze and to understand the fracture performance of polycrystals at the meso- and microscopic level. Nowadays, the phase-field method has established as one of the promising tools for the description of crack propagation in different kinds of materials [1, 2]. In particular, the multiphase-field theory is a perfect tool to study the crack propagation in polycrystalline media, because the interface energy between any pair of grains and surface energy are the natural input parameters for the model [3]. The main purpose of this study is to develop a comprehensive multiphase-field model for fracture in a multiphase system allowing for hydrogen diffusion. Theoretical analysis is complemented with developments in computational modelling of the simultaneous fracture and diffusion phenomena. The model is implemented inside the open source software project OpenPhase to solve some examples in three-dimensions. The model reproduces well different cases. It is shown that how the hydrogen affects the competition between intergranular and transgranular cracking. [1] H. Jafarzadeh, G. H. Farrahi, V. I. Levitas, and M. Javanbakht, Phase field theory for fracture at large strains including surface stresses, Int J Eng Sci 178 (2022) 103732-103760. [2] H. Jafarzadeh, V. I. Levitas, G. H. Farrahi, and M. Javanbakht, Phase field approach for nanoscale interactions between crack propagation and phase transformation, Nanoscale 11 (2019) 22243-22247. [3] I. Steinbach, Phase-field models in materials science, Model Simul Mat Sci Eng 17 (2009) 073001– 073031.