The present work shows how highly tough hydrogels can be prepared through a simple freeze-thaw-annealing approach and can be organized into robust Fenton reactors for in situ catalytic degradation of organic dyes. Hydrogels, as a kind of tissue-like materials, have been of great interest in recent decades. However, their underwhelming mechanical properties dramatically limit their applications. Here, highly tough hydrogels are obtained through a simple freeze-thaw-annealing approach using polymer modules, and are fabricated into Fenton reactors for in situ catalytic degradation of organic dyes. The associated polymer modules are poly(vinyl alcohol) (PVA) and poly(acrylic acid) (PAA). Upon a freeze-thaw-annealing treatment on the mixture of PVA and PAA, well-organized PVA crystal structures and unexpected ester groups between PVA and PAA are formed, resulting in highly tough hydrogels rivaling biological tissues. Moreover, upon the addition of Fe3+ further reinforced hydrogels were obtained, which remain intact in aqueous media even with vigorous stirring. As a proof-of-concept, these Fe3+-loaded tough hydrogels are molded into renewable Fenton reactors for in situ catalytic degradation of organic dyes. This study not only shows a simple approach toward highly tough hydrogels but also suggests a new concept of soft material-based chemical reactors.