Hydrogels can be used to prolong the retention of photothermal agents in tumors to conduct repeated photothermal therapy (PTT) and thus reduce high-dose-induced toxicity, but the long-term retention of photothermal nanoparticles post-therapy still increases the risk of toxicity. In this work, we developed an injectable and degradable photothermal hydrogel to conduct repeated PTT. The hydrogel was composed of dendrimer-encapsulated platinum nanoparticles (DEPts) crosslinked aldehyde-modified dextran via imine bond formation. The hydrogel represented excellent photothermal effect and good biocompatibility. It was able to remain in tumors for a period of days to allow repeated PTT, leading to complete tumor regression. After treatment, the hydrogel was gradually degraded due to the decomposition of imine bonds. The study developed a practical photothermal hydrogel that allowed repeated PTT and reduced long-term retention-induced toxicity.