In this study, we developed novel thermal and redox-responsive micelles based on the Pluronic F127 tri-block copolymer and employed these for redox-responsive intratumor release of bufalin, an anti-cancer drug. Pluronic F127 was first functionalized with carboxylate groups, and then assembled into micelles. The HOOC-F127-COOH micelles are 20 ± 4 nm in size at 37 °C, but expand to 281 ± 5 nm when cooled to 4 °C. This allows for the free diffusion of bufalin into the micellar cores at low temperatures, while at 37 °C the micelles are much more compact and the drug molecules can be effectively held in their interiors. A high encapsulation efficiency and loading content were obtained via drug incorporation at 4 °C. The drug-loaded micelles were cross-linked with cystamine, which contains a disulfide bond responsive to the local cancer microenvironment. In vitro studies showed that drug release from the cross-linked micelles was low under normal physiological conditions, but markedly accelerated upon exposure to conditions representative of the intracellular tumor environment. Confocal microscopy revealed that the cross-linked micelles gave high levels of drug release inside the cells. In vivo studies in mice showed the drug-loaded cross-linked micelles have potent anti-tumor activity, leading to high levels of apoptosis of tumor cells and significant reductions in tumor volume. The drug-loaded cross-linked micelles did not significantly influence body weight, and there was no evidence for detrimental off-target effects. These results indicate that the Pluronic-based micelles developed in this work are promising drug delivery systems for the targeted treatment of cancer.