This paper presents a series of consolidated-undrained triaxial compression tests for investigating the shear behavior of geotextile-reinforced clay and generation of excess porewater pressure during undrained loadings. Specimens were prepared at their maximum dry unit weight and optimum moisture content, and the effects of the confining pressures and the number of geotextile layers were investigated. The experimental results revealed that during consolidation, nonwoven geotextile as a permeable material reduced the time of consolidation; however, it induced a higher volume change. During undrained loading, the shear strength and excess porewater pressure of the reinforced clay increased with the number of geotextile layers because of the restraint of the lateral deformation resulting from the mobilized tensile force of reinforcement layers. Both effective and total stress failure envelopes of the reinforced clay shifted upward as the number of reinforcement layers increased and appeared to be parallel to those of the unreinforced clay. Modified porewater pressure parameters, A* and B*, for reinforced clay were proposed using the additional confinement approach. The parameters A* and B* can be used to quantitatively evaluate the influence of soil and reinforcement on excess porewater pressure generation, respectively. The A* value is close to Skempton's porewater pressure parameter A for unreinforced soil. The B* is defined as the ratio of porewater pressure difference to additional confining pressure. The lower B* value indicates that the reinforcement is more effective in enhancing additional confining pressure than increasing excess porewater pressure. This study demonstrated that the effect of geotextile layers on inducing additional confinement was more marked than that on the generation of excess porewater pressure, resulting in an increase in effective confining pressure and subsequently in the shear strength of reinforced clay.