Advancing cancer research depends significantly on developing accurate and reliable models that can replicate the complex tumor microenvironment. Tumor spheroids─three-dimensional clusters of cancer cells─have become crucial tools for this purpose. The overarching goal of tumor spheroid culture is to develop biomaterials that mimic the dynamic mechanical behavior of the native extracellular matrix, enabling high-fidelity culture models. In this study, we developed dynamic hydrogels based on dual-dynamic covalently cross-linked polyglycerol, using boronate bonds and Schiff-base interactions. In addition to good biocompatibility and long-term stability, the hydrogels showed tunable mechanical properties that enabled cells to actively remodel their surrounding microenvironment. This platform was used for successful 3D culture of various cancer cell lines, including HeLa, A549, HT-29, BT-474, and SK-BR-3, which were encapsulated in situ and formed 3D tumor spheroids. These results demonstrate the feasibility and versatility of our dynamic hydrogel system in supporting tumor spheroid culture.