This study compares the differences in the mismatch between the education and occupations of immigrants in Canada and the United States, operationalized by over-education. It further explores how the cross-country differences may be related to the supply of and demand for university-educated immigrants and the way they are selected. Using comparable data and three measures of over-education, this study found that university-educated recent immigrants in Canada were much more likely to be overeducated than their U.S. peers. The over-education rate gap between recent immigrants and the native-born was much more pronounced in Canada than in the United States. In addition, while labour market demand was associated with a lower level of over-education in both countries, a greater supply of university-educated recent immigrants was positively associated with a likelihood of over-education among recent immigrants in Canada, but not in the United States. Furthermore, in Canada, the over-education rate was significantly lower among immigrants who were admitted through some form of employer selection (e.g., immigrants who worked in skilled jobs in Canada before immigration) than those who were admitted directly from abroad. Overall, this study provides insight into how the immigration system interacts with broader aspects of the labour market to shape the labour market outcomes of immigrants.