A new reactive urethane–silicone softener was developed to provide a soft touch to cotton knit fabrics with improved durability to washing and dimensional stability. The reactive urethane–silicone softener consisted of an amino silicone softener and a blocked isocyanate, which can crosslink and react with cellulose surfaces. The activated isocyanate from the blocked isocyanate reacted with the amino silicone softener by heat treatment at 150 °C for 30 min. The mechanical properties of the cotton knit fabrics treated with the urethane–silicone softener were evaluated using a Kawabata Evaluation System-Fabrics (KES-FB) system. The cotton knit fabrics treated with the urethane–silicone softener showed excellent elasticity, flexibility and shear recovery as well as excellent recovery against bending deformation, and soft and smooth surface characteristics with a small coefficient of friction that were maintained even after washing 20 times.