<p>Soft biological tissues exhibit mechanical properties that reflect their composite structure of cells embedded within a biopolymer matrix. However, the microscopic mechanisms underlying their unique nonlinear mechanical response—characterized by strain stiffening in compression, but strain softening in shear or tension—remain poorly understood. Here we show that strain softening in composite systems can arise due to plastic dissipation, which is mediated by filler–polymer interactions. We characterize the nonlinear elasticity of composite hydrogels and soft tissues in isolation from these plastic effects, and show that their nonlinear elastic strain stiffening is driven by the stretching of the underlying biopolymer matrix. We thus show that strain stiffening in composite hydrogels and tissues is mediated by strain amplification factors that are universal in compression and shear. In doing so, we demonstrate the importance of fundamental composite properties such as filler concentration and filler–polymer interaction strength in mediating strain stiffening in composite systems. These findings highlight key structure–property relationships that underlie the nonlinear mechanics of biologically relevant soft solids such as composite gels and tissues.</p>