The international crop seed trade is a major pathway for the unintentional introduction of non-native invasive plant species and herbicide-resistant weeds, posing biosecurity threats to agriculture and ecosystems. However, published studies examining weed contaminants in crop seed remain scarce. To address this, we analysed Canadian Food Inspection Agency (CFIA) monitoring data for 2,080 randomly sampled crop seed lots imported from the United States of America (USA) between 2007 and 2019. Both nations are major players in the global seed trade, making them key biosecurity case studies. We reported 262 contaminant species: 70% were introduced in Canada, 23% were native, and 7% had not been previously recorded (absent) in Canada. General weeds (not also imported as crops) comprised 63% of contaminant species; the remaining species were classified as seed of another crop. CFIA-classified noxious weeds (Classes 1–5) made up 12% of the contaminant species. Most contaminants were associated with only one or two crop species. There was a decline in general and noxious weeds, and noxious weeds were reported significantly less than non-noxious weeds over the study period. Entry-prohibited species (Class 1) were rare, limited to four records of Cuscuta spp. We identified 14 general weed species currently absent from Canada, notably the frequently reported Trifolium vesiculosum, along with Galium parisiense, Torilis nodosa, and Trifolium hirtum, all established in climatically similar regions of the USA, as well as Bromus catharticus and Euphorbia aleppica, identified as environmental and agricultural threats. Eight additional species, such as Apera spica-venti, currently limited to one Canadian province, pose a potential domestic spread risk. Reported Class 2 CFIA noxious weeds, including Cirsium arvense, Convolvulus arvensis, and Elymus repens, are of concern as their ecological range is not fully realised in Canada. Chenopodium album was the most widespread general weed detected across crop species. Contaminants with a known history of herbicide resistance in the USA but not in Canada increased significantly over time (e.g., Sorghum halepense, Poa annua), while those resistant in one Canadian province (Bromus tectorum) risk further spread in Canada. The introduction of new resistance is of concern when a contaminant species is reported in a crop type in Canada and documented as herbicide-resistant in the same crop type in the USA (Poa annua in forage and turf seed lots). Regulatory concerns include importing crops that are also classified as noxious or problematic weeds (Bromus tectorum, Poa annua), permitting contaminants absent from Canada in seed lots, and the dual classification of species that are native but also entry-prohibited (Cuscuta campestris). Our study highlights that expanded global seed trade necessitates ongoing seed lot monitoring, risk assessment, and adaptive regulations to help safeguard agriculture and biodiversity without hindering trade.