In the context of underground works excavated in rock, their long-term behaviour and stability are conditioned by the time-dependent behaviour of the surrounding rock (Armand et al., 2017). Considering clay rocks, and due to their relatively high clay mineral content, there is a necessity of accurately modelling the delayed viscous behaviour and creep strain of the rock. This study describes the time-dependent mechanical behaviour of clayey rocks by introducing a micro-mechanics based viscoplastic model of the clay matrix. This model is considered within a two-scale finite element framework. The non-homogeneous rock is represented at the microscale as a composite material consisting of rigid elastic mineral inclusions embedded in a clay matrix. Mesotructures of the clayey rock are generated in 2D representative elementary areas (REAs) in which several microscale morphological and physical characteristics of the rock are numerically described (van den Eijnden et al., 2017; Pardoen et al., 2020). To describe the damage and failure modes at small scale, interfaces between the different mineral phases and within the clay matrix are considered as potential microcracks. They are modelled by a damaged cohesive model. The viscoplastic constitutive law of the clay matrix is an extension of Lemaitre’s creep model incorporating mean stress-dependency and a non-associated flow rule. The creep model developed at small scale (micro and meso scales) is applied to the Callovo-Oxfordian (COx) claystone in order to model its large-scale creep behaviour at laboratory scale. A clear three-stage creep process is reproduced, including the primary creep stage, second creep stage, and tertiary creep stage. It is found that the developed multiscale model is able to provide valuable insights into the large-scale creep behaviour of clay rocks through the morphological and material small-scale characterisation of REA.