Growth of the nuclear industry has encouraged us to look for techniques to treat large volumes of nuclear waste. 60Co is one of the most problematic radioactive wastes in the nuclear industry. In this study, a Mn slag-based geopolymer (MSG) was prepared, which exhibited better Co immobilization performance than the ordinary metakaolin-based geopolymer (MKG). Varying the mass ratio between water glass and NaOH (mw/mn) influenced the structure and chemical performance of the MSG samples, which consequently influenced their Co immobilization capacity. The optimized MSG was obtained at an mw/mn value of 1.5. After the 7-day leaching test, about 0.20% of the Co was released from MSG, which is less than two-thirds of the MKG sample. The chemical state of Co in the geopolymer matrix was characterized by X-ray photoelectron spectroscopy. Divalent Co remained in the MKG samples, while most of Co ions in MSG samples existed in the trivalent state. These results strongly suggest that divalent Co was oxidized to trivalent Co in the MSG matrix, resulting in enhanced Co solidification capacity compared to MKG. The results in this study indicate that the oxidation environment in the MSG played an important role in Co immobilization. Display omitted •A Mn slag-based geopolymer (MSG) was fabricated to immobilize Co.•The chemical performance of MSG is affected by the water glass/NaOH mass ratio.•Co released from MSG was much lower than the ordinary metakaolin-based samples.•The oxidation environment in MSG played an important role in Co immobilization.