Objective: The existing theoretical approaches to group flow deviate from each other and have several limitations. Given that the assumptions of the existing approaches are not incompatible with each other and instead can be regarded as being complementary, the purpose of this article is to propose the integrative group flow theory (IGFT) as a theory that integrates and extends existing approaches. Results: The IGFT is based on action theoretical principles and describes group flow as balanced group action. Balanced group action is a continuously perfectly fitting handling of the task by the group system in a given situation. This involves relationships within and between the three action theoretical components of group action (i.e., group system, group task, group environment) and three major functions of the group system (i.e., state of mind, behavior, skills). Group flow is described as dynamic due to changes of the group situation and due to regulatory processes within the group, particularly interpersonal coordination. Conclusion: In terms of systematics, the IGFT meets the goodness criteria (i.e., comprehensiveness, parsimony, openness). Moreover, the theory also meets specific recommendations for the development of theories for group phenomena. In terms of empiricism, the results of the existing studies already confirm many aspects of the IGFT. Further research is needed to test the remaining aspects. This requires the development of a measurement instrument and an experimental paradigm. The theory can be expected to fulfill the functions of scientific theories (i.e., epistemology, praxeology, communication).