Expression of a high concentration of H2S is closely related to the formation of colon cancer tumors. However, only a few H2S‐triggered theranostics agents for colon cancer have been reported to date. Now, a turn‐on theranostics agent was developed by utilizing the in situ reaction of Cu2O and endogenous H2S at colon tumor sites. Based on in vitro and in vivo experiments, excellent photoacoustic imaging and photothermal therapy were both confirmed by this in situ reaction‐activated colon cancer theranostics method. This work established a simple and efficient strategy for both diagnosis and treatment of colon cancer with a novel trigger mechanism, which provides a new route for colon cancer theranostics based on the in situ reactions at the tumor sites. A turn‐on theranostics agent was developed by utilizing the in situ reaction of Cu2O (see picture: brown) and endogenous H2S (yellow) at colon tumor sites. Based on in vitro and in vivo experiments, excellent photoacoustic imaging and photothermal therapy were both confirmed after being triggered by this in situ reaction. Cu9S8 shown in green.