The use of soil-fiber has been recognized as a viable technique of soil improvement in numerous geotechnical applications. However, existing research has not yet established the fundamental mechanisms controlling the behavior of clay-fiber mixtures and the constraints that may affect their performance. On the other hand, there is a growing increase in the development of methodologies for predicting soil-fiber behavior, as well as the use of soil-mixture constitutive models. In this context, the present work aims to contribute to the understanding of shear strength and deformability behavior of a clay soil reinforced with recycled polypropylene (R-PP) short fibers under drained and undrained triaxial tests. In terms of effective stresses, in drained condition, results showed that the fibers caused an increase in soil cohesion, remaining the angle of friction practically unaffected. In undrained condition, the contribution of fibers on friction angle increase was more significant than cohesion increase. Drainage during shearing have favored soil-fiber shear strength. All fiber-soil specimens presented “bulging” behavior after shearing. The interaction mechanisms between the soil and fibers, and consequently the shear strength of the composite, were not influenced by the drainage condition or by the development of pore water pressures during shearing.