There is evidence showing that the triarchic model of grit and its dimensions (i.e., "perseverance of effort," "consistency of interests," and "adaptability to situations") predict engagement and well-being outcomes in high school and undergraduate students during the COVID-19 pandemic outbreak. However, there has been limited research on how this model of grit relates to engagement and optimal psychological outcomes in primary school students. This research investigates the association of grit's dimensions with academic engagement in math and science as well as well-being outcomes (i.e., positive emotions, negative emotions, and flourishing) in primary school students. Participants were primary school students (M[subscript age] = 10.42; SD[subscript age] = 1.26) from Hong Kong (n = 279) and Macau (n = 124). Results showed that "perseverance of effort" positively predicted cognitive and behavioral engagement in math as well as positive emotions even after controlling for demographic covariates (i.e., age, gender, setting, and year level), conscientiousness, and achievement goal orientations. "Adaptability to situations" positively predicted cognitive and social engagement in math and flourishing. "Consistency of interests" negatively predicted both cognitive engagement in science and negative emotions. Indeed, this study indicates that "perseverance" and "adaptability" may facilitate children's positive academic and psychological functioning during the COVID-19 pandemic outbreak.