Provided are: an exhaust gas treatment catalyst capable of improving NO conversion rate when performing denitrification using CO as a reducing agent, and improving CO oxidation rate when oxidizing CO present in the exhaust gas; a method for producing an exhaust gas treatment catalyst; and an exhaust gas treatment system. The exhaust gas treatment catalyst is a catalyst which uses CO as a reducing agent to treat exhaust gas from a sintering furnace, and contains: a support that is a metal oxide or metal sulfate; and an active metal containing at least iridium supported by the support, wherein the specific surface area of the catalyst is 100 m2/g or less, and the crystallite size of iridium in the catalyst is 10-25 nm.