Fricke gel dosimeters are based on the aqueous ferrous sulfate solution that has been used as a reliable chemical dosimeter for more than 90 years. The amount of radiation-induced oxidation of ferrous ions to ferric ions could be evaluated three-dimensionally via optical computed tomography (OCT) or magnetic resonance imaging (MRI). Three-dimensional dosimetry is expected to be a useful tool, in particular, for the veriﬁcation of complex radiation dose planning in advanced radiation cancer therapy. Compared with other 3D gel dosimeters, there are several problems such as retention of dose distribution or dose sensitivity; however, they can easily be prepared in a chemistry laboratory. In addition, a unique gel dosimeter was also reported for heavy-ion beam irradiation. In this review, recent papers concerning the Fricke gel dosimeter and its application in 3D dosimetry are summarized.