Redox-responsive hydrogels have the potential for application in various fields, including biomedical science. We have developed a redox-responsive star block copolymer hydrogel based on iron ion cross-linking. The addition of the ferric ion (Fe3+) induced gelation of the star block copolymer solution within a few seconds, whereas the addition of the ferrous ion (Fe2+) did not. The resulting hydrogels cross-linked using Fe3+ showed storage moduli (G') of 26-1400 Pa and were stable under physiological conditions for as long as 1 month. The cross-linking between the star arms produced by the addition of Fe3+ enabled a fast, redox-responsive sol-gel/gel-sol transition. Furthermore, the hydrogel showed excellent injectability and biocompatibility in vivo, resulting in a rapid sol-gel/gel-sol transition in subcutaneous tissues in response to redox stimuli, such as the administration of ascorbic acid or hydrogen peroxide.