In both Canada and the United States, immigrants constitute a disproportionately large share of the supply of university-educated labour trained in the science, technology, engineering and mathematics (STEM) fields. This article examines the Canada-U.S. differences in the occupational skill utilization and earnings of STEM-educated immigrant workers. Using data from the 2016 Census for Canada and the combined 2015 to 2017 American Community Survey, this analysis focuses on immigrants with a university degree in a STEM field who were aged 25 to 64 and arrived as adults. Over one-half of STEM-educated immigrant workers in both countries held non-STEM jobs. In Canada, only about 20% of these immigrants with non-STEM jobs worked in occupations that required a university education, compared with 48% in the United States. There was a large earnings gap between STEM-educated immigrants and native-born workers in Canada, even after adjusting for sociodemographic differences, while no corresponding earnings gap existed in the United States. The earnings gap in Canada was particularly large for STEM-educated immigrants holding non-STEM jobs. Possible explanations for these results are discussed.