Published laboratory geotechnical data by Masui and co-authors showed that increase in gas hydrates content tend to increase the peak shear strength and stimulate strain softening of the host sediment. Therefore, development of shear strains may lead to an important degradation of the shear strength (strain softening). In the present work, the strain softening of gas hydrate-bearing sediments was implemented in a 3D slope stability model (SAMU-3D). This was done by adding to the classical limit analysis method a shear strain field compatibility equivalent to the velocity field compatibility. Examples of slope failures related to strain softening behavior documented in the literature were used to test the model formulation. The developed model was then used to assess the stability of a steep flank of a shale-cored anticline in the eastern part of the offshore Niger delta. Numerical modeling showed that the formation of gas hydrates in the shallow sedimentary layers could considerably affect the factor of safety of the studied slope. The present work showed clearly that the strain-softening behavior of gas hydrate-bearing sediments has relevance for the stability of submarine slopes.