Children’s happiness is an important issue that needs to be addressed around the world; however, limited research has explored how children define happiness. In this study, we conducted a survey of elementary school children living in different countries and clarified the contextual definitions of happiness by a total of 842 children (Mean age = 11.01, SD = 1.05, effective rate = 97.34%) from Grades 4 to 6 living in Japan (n = 474), Mongolia (n = 121), and Finland (n = 247). We investigated the definition of children’s happiness using the stimulus sentence completion test. For Mongolian children, happiness was predominantly characterized by references to family relationships. For Finnish children, happiness appeared to be linked to autonomous involvement with things, personal achievement, and success. Furthermore, the happiness of Japanese children was largely characterized by pleasant versus unpleasant experiences. In addition, Japanese children often referred to “being scolded” and their happiness was not related to school success. Our findings concurred with the elements of Seligman’s (2012) concept of flourishing as a new criterion for judging well-being. It was also suggested that the dominant elements of happiness may differ depending on an individual’s country and culture.