Social robots can be designed to facilitate co-creation. The impact of mood expressions displayed by human facilitators can elicit similar moods in others, fostering collaboration, process satisfaction, and co-creation performance. However, the extent to which robot mood expressions can influence such group dynamics to the same effect remains an open question. To address this, we conducted an experiment (N = 110) in which small groups engaged in a co-creation session facilitated by a social robot that displayed positive, neutral, or negative mood expressions. The results showed that robot mood expressions evoked corresponding mood contagion among human group members. The influence of robot expressions on mood valence significantly enhanced group collaboration and process satisfaction. Additionally, participants’ ability to contribute insights during the session and ensure these are represented in session outcomes, an index of co-creation performance, significantly improved when positive robot mood expressions increased mood contagion valence. By establishing the influence of robot mood expressions on mood contagion, group collaboration, process satisfaction, and co-creation performance, we shed light on the potential of social robots for facilitating and enhancing co-creation. Understanding the mechanisms underlying these effects adds to a growing body of work in the field of human-robot interaction and offers valuable insights for developing innovative future robotic systems.