Feral rapeseed in Japan consists of Brassica rapa, B. juncea, and B. napus, mostly produced by escape from crops. Brassica rapa and B. juncea were introduced from abroad long ago as leaf and root vegetables and as an oil crop, and breeders have developed various cultivars. Brassica napus was introduced in the late 1800s, mainly as an oil crop. Rapeseed production in Japan is low, and most demand is met by imports from Canada (94.4% of the 2009 trade volume). Recently, spontaneous B. napus, including genetically modified (GM) herbicide-resistant individuals, has been detected along Japanese roads, probably originating from seeds lost during transportation of imports. As GM oilseed production increases abroad, the probability of escape of GM oilseed rape in Japan will increase, raising environmental biosafety concerns related to the impact of feral rapeseed on heirloom brassicaceous crops. In this paper, we review the history of rapeseed introduction in Japan and future concerns.