Ill-structured problem solving is widely believed to promote learning in higher education but its multiplicity (i.e. multiple solutions and evaluation criteria) is often seen as a challenge to manage. This study shows that the multiplicity can be managed as well as leveraged to enhance learning effectiveness through online co-creation. Three co-creation activities are identified and their effects on different aspects of learning effectiveness are assessed in a study involving 225 tertiary-level students. Results indicate that solution co-creation and solution sharing enhance cognitive learning (e.g. perceived knowledge about a subject topic), while decision co-creation enhances epistemic learning (e.g. perceived understanding of criteria of knowing). The findings demonstrate the value of online co-creation and they pave the way for more research on online co-creation in other collaborative pedagogical practices such as co-skilling.