Sea cucumbers own the fascinating capability to rapidly and reversibly change the stiffness of their dermis. This mechanical morphing is achieved through a distinctive architecture of the tissue, which is composed of a viscoelastic matrix that is reinforced with rigid collagen microfibrils. Neurosecretory proteins regulate the interactions among the latter, and thereby control the overall mechanical properties of the material. This architecture and functionality have been mimicked by researchers in artificial nanocomposites that feature similar, albeit significantly simplified, structure and mechanical morphing ability. The general design of such stimulus–responsive, mechanically adaptive materials involves a low-modulus polymer matrix and rigid, high-aspect ratio filler particles, which are arranged to form percolating networks within the polymer matrix. Stress transfer is controlled by switching the interactions among the nanofibers and/or between the nanofibers and the matrix polymer via an external stimulus. In first embodiments, water was employed to moderate hydrogen-bonding interactions in such nanocomposites, while more recent examples have been designed to respond to more specific stimuli, such as a change of the pH, or irradiation with ultraviolet light. This chapter provides an overview of the general design principles and materials embodiments of such sea-cucumber inspired materials.