Mechanical deformation of polymer networks causes molecular-level motion and bond scission that ultimately lead to material failure. Mitigating this strain-induced loss in mechanical integrity is a significant challenge, especially in the development of active and shape-memory materials. We report the additive manufacturing of mechanical metamaterials made with a protein-based polymer that undergo a unique stiffening and strengthening behavior after shape recovery cycles. We utilize a bovine serum albumin-based polymer and show that cyclic tension and recovery experiments on the neat resin lead to a ~60% increase in the strength and stiffness of the material. This is attributed to the release of stored length in the protein mechanophores during plastic deformation that is preserved after the recovery cycle, thereby leading to a “strain learning” behavior. We perform compression experiments on three-dimensionally printed lattice metamaterials made from this protein-based polymer and find that, in certain lattices, the strain learning effect is not only preserved but amplified, causing up to a 2.5× increase in the stiffness of the recovered metamaterial. These protein–polymer strain learning metamaterials offer a unique platform for materials that can autonomously remodel after being deformed, mimicking the remodeling processes that occur in natural materials. Biological materials such as bone and spider silk undergo stress-induced remodeling processes that enhance their mechanical properties. Bone can form stiffness gradients in response to force mechanotransduction ( 1– 5) and unspun spider silk proteins are under constant shear stress during the fiber-forming process to enhance the strength of the filaments ( 6– 11). Engineered materials that can mimic the stress-responsive features of these natural systems could enable autonomous materials that strengthen in response to mechanical cues from the environment for applications in the aerospace and medical fields ( 12). However, traditional synthetic thermosets are limited in their ability to adapt to mechanical forces, and in many cases, the materials become weaker or more brittle after mechanical deformation ( 10). Bonds cleavable via homolytic scission or dynamic covalent reactions can provide mechanisms for stress relaxation, but these materials do not necessarily afford a thermoset with improved mechanical strength ( 13). Alternatively, reactive strand expansion also provides a molecular-scale mechanism for dissipating stress within a network by utilizing polymers that can release their “stored length” ( 14– 19). All of these approaches are effective for mitigating stress and damage within a cross-linked polymer network, but to the best of our knowledge, have not led to stress-induced enhancement of mechanical properties that is analogous to biological systems.