Gels represent an increasingly important class of soft materials with applications ranging from regenerative medicine to commodity materials. However, gels typically exhibit relative mechanical weakness, which worsens under repeated strain. Here we report a new class of responsive gels with latent crosslinking moieties that exhibit strain-stiffening behavior. This property results from the lability of disulfides, initially isolated in a protected state, then activated to crosslink on-demand. The thiol groups are induced to form inter-chain crosslinks when subjected to mechanical compression, resulting in a gel that strengthens under strain. Molecular shielding design elements regulate the strain-sensitivity and spontaneous crosslinking tendencies of the polymer network. These strain-responsive gels represent a rational design of new advanced materials with on-demand stiffening properties and potential applications in elastomers, adhesives, foams, films, and fibers.