Creativity mindset (CM), grit, and self-determination have been defined as critical motivational variables affecting learners’ self-efficacy. Therefore, this study pioneers the examination of the relationship between these motivational variables and creativity self-efficacy (CSE) during game-based learning. A Creativity Mindset Inventory (CMI) and a game-based learning intervention were employed. Participants for developing the CMI were 281 3rd to 6th graders, and those for the intervention were 114 3rd and 4th graders. The result revealed that the CMI included four constructs (growth-internal control, growth-external control, fixed-internal control, and fixed-external control). Moreover, the employed intervention enhanced the children’s growth CM and CSE. Regression analysis results suggest that self-determination mediates the influence of growth CM and grit on CSE. Additionally, growth CM, especially the growth-internal control CM, is a powerful predictor of self-determination and CSE. In contrast, fixed CM (the overall fixed CM, the fixed-internal control CM, or the fixed-external control CM) does not have any significant influence on self-determination or CSE. Notably, the findings of this study support that growth CM can be enhanced through a well scaffolded educational game. This study contributes to the field of game-based learning by developing a CM inventory, demonstrating a growth CM intervention, and clarifying influential factors to CSE during game-based training. While game-based learning has become popular among elementary school students, the findings of this study provide important insights into the design of game-based learning and creativity training.