The present investigation aimed to understand the physicochemical properties of the new cellulosic fiber extracted from the bark of Silybum marianum (SM), in view of using it as a potential reinforcement for polymer composites. The morphological and anatomy, physical, thermal and mechanical properties of fibers were firstly discussed in this paper. The Silybum marianum fibers (SMF) were characterized by scanning electron microscopy, Fourier transform infrared, thermogravimetric analysis (TGA), optical microscope, X-ray diffraction (XRD), and single fiber tensile test. The average Young’s modulus and the breaking stress data presented by the fibers are 15.97 GPa and 201.16 MPa, respectively. XRD reveals the presence of cellulose with a crystallinity index of 45%. Thermal stability (250°C) and maximum degradation temperature (357.72°C) of the SMF are established by the thermogravimetric analysis. An analysis of the mechanical properties was carried out on a population of 35 samples using Weibull statistics with two and three parameters.