The aim of this study was to examine the relationship between motor skills proficiency and participation in physical activity considering three mediators (ability beliefs, subjective task value, and expectancy of success) among Grade 3 children and considers those relationships in terms of sex-based differences. The participants in this study were recruited from eight elementary schools from School District 61 in Victoria, British Columbia. Participants were 398 children (Girls: n = 201, Boys: n = 197). Motor skills were assessed using the Test of Gross Motor Development-2 (TGMD-2), physical activity participation was measured using the Children’s Assessment of Participation and Enjoyment (CAPE), and ability beliefs, subjective task value, and expectancy of success were measured using the Expectancy Value Questionnaire (EVQ). Descriptive statistics showed that participation in physical activities was low with a mean score of 3.7 on a scale of 14. Percent of maximum (POMP) scores of the psychological variables were in the middle of the range of possible scores; specifically: 68.7%, 74.8%, and 72.7% for children’s ability beliefs, task value, and expectancy of success, respectively. A MANCOVA revealed a main effect of sex (F (7, 389) = 29.684, p < .001; Wilks’ Lambda = 0.652) between boys and girls in terms of their ability beliefs, expectancy of success, subjective task value, motor skills proficiency. A second MANCOVA examining the effect of sex on total raw scores of motor skills and physical activity also revealed a main effect of sex (F (2, 394) = 11.130, p < .001; Wilks’ Lambda = 0.947). Separate parallel multiple mediator models were created for both boys and girls. The mediator model for boys revealed an overall significant effect of .044 (p < .001). The mediator model for girls revealed an overall significant effect of .031 (p < .05). The mediation model for boys showed that the psychological variables in this study did not mediate the relationship between motor skills and physical activity participation. Instead, boys’ motor skills directly predicted their participation in physical activity. The girls’ mediation model showed mediation between motor skills and physical activity with subjective task value as the mediator. Girls’ motor skills did not have a direct relationship with their participation in physical activities. Future research might: (1) include gender as a mediating factor in future mediation models, (2) explore mediation models with locomotor skills and object control skills as independent variables, and (3) explore the role of social and environmental factors such as the influence of parents, teachers, peers, culture, and society on children’s participation in physical activity. / Graduate