This study was to design a novel wide pH–sensing colorimetric film of κ-carrageenan incorporated with extract from Lycium ruthenicum Murr. (Car-LRM) for food freshness monitoring. The structure, basic properties (thermal stability, mechanical, barrier and antioxidant activity), and release kinetics of LRM from the films were investigated. Attentions were centered on colorimetric response of films and their applications for freshness monitoring of milk and shrimps. A low incorporation of LRM (no more than 2.5%) led to an enhancement in thermal stability and water vapor barrier of films to a certain extent. At above level of incorporation, LRM could be well-distributed in Car matrix by the interactions of hydrogen bonds. In addition to good antioxidant activity, the film displayed colorimetric response at a wide range of pH (2−10) and the color change was reversible. The release of LRM from films followed Fickian's law which was controlled by diffusion mechanism rather than swelling mechanism. The successful applications of Car-LRM2.5 for the freshness monitoring of milk and shrimps indicate that Car-LRM2.5 may have great potential for monitoring the freshness of milk and aquatic products. Display omitted •Development of a wide pH–sensing colorimetric film based on κ-carrageenan.•Colorimetric film exhibits good release control and antioxidant activity.•Fickian diffusion is the predominant mechanism of release.•Application as an indicator for freshness monitoring of milk and shrimps.