We explore the effect of higher-form symmetry on thermalization. We analytically demonstrate that, under certain assumptions, the presence of higher-form symmetries leads to a breakdown of the Eigenstate Thermalization Hypothesis (ETH) for nontrivial observables. For discrete symmetry groups, the breakdown of the ETH is attributed to the presence of non-local conserved quantities. For a p-form symmetry, the operators that break ETH become \((d-p)\) -dimensional. These operators are non-local but significantly smaller in size compared to the entire system for \(p \ge 1\) . We illustrate this for the two-dimensional \(\mathbb {Z}_2\) lattice gauge theory with \(\mathbb {Z}_2\) 1-form symmetry. Additionally, while local observables relax to the canonical ensemble, a non-local operator that excites a magnetic dipole relaxes to the Generalized Gibbs Ensemble (GGE) that accounts for the higher-form symmetry. Our findings suggest that such symmetries lead to nontrivial thermalization processes detected by non-local observables, extending beyond conventional statistical mechanics.