For almost a hundred years Japanese fighting men were employed across a wide area of Southeast Asia as mercenaries in the service of the kings of Siam, Cambodia, Arakan, Spain and Portugal and for the directors of the Dutch East India Company. This activity is described and discussed in the context of contemporary international relations and the spread of European colonialism in the region, noting in particular the impact on the recruitment and activity of mercenaries caused by Japan’s progressively strict regulations from 1621 onwards regarding the employment of its citizens, their overseas travel and Japan’s relations with neighbouring countries. The Japanese ‘Wild Geese’ (a colloquial term for mercenaries) were used in dramatic assault parties, as staunch garrisons and as willing executioners. A stereotypical image of the fierce Japanese warrior thereby developed that had a profound influence on the way they were regarded by their employers. Although this image, which began with the wakō (Japanese pirates,) was a positive one at first, their reputation acquired a negative sheen when employers experienced difficulties in disciplining them or began to question their loyalty. Two contrasts will be noted. First, the mercenaries’ role with regard to the rulers of Southeast Asian kingdoms was largely that of acting as long-term palace guards, while recruitment by European powers tended to be of shorter duration for specific campaigns. Second, whereas the Southeast Asian monarchs tended to trust their well-established units of Japanese mercenaries the Europeans admired them but also feared them, and in every European example a progressive shift in attitude may be discerned from initial enthusiasm to great suspicion that the Japanese might one day turn against them, as illustrated by the long-standing Spanish fear of an invasion of the Philippines by Japan accompanied by a local uprising. Ironically this fear was matched by a similar concern on the other side that Spain would invade Japan with the assistance of Japanese Christians who had been exiled and a ‘fifth column’ of Christian sympathisers within Japan itself. The part played by the suppression of Japanese Christianity in the creation of the independent groups of exiled warriors who posed such a threat is also discussed, and suggestions are made as to the reasons for the profound change of policy with regard to the employment of Japanese mercenaries which took place under the Shogun Tokugawa Hidetada. It is finally suggested that if, during the 1630s, Japan had chosen engagement with Southeast Asia rather than isolation from it the established presence of Japanese communities overseas may have had a profound influence on the subsequent development of international relations within the area.