The aim of this paper is to report the colonization process of three headwater rivers of two large trans-national Pacific river basins shared by Chile and Argentina, by exotic Chinook salmon (Oncorhynchus tshawytscha). To characterize the populations at the sites, fish were analyzed in terms of length, weight, age, and stream residency patterns. Fishermen familiar with the area were interviewed to establish the seasonal occurrence of fish and to reconstruct the history of the colonization process. Records of salmon production in Chile by regions were examined to evaluate the potential origin of fish in coastal net pen aquaculture. The regularity of spawning runs in our study sites indicates that Chinook salmon have established spawning populations in these rivers. While spawning was verified in all three rivers, only one seems to serve as a holding area for pre-spawning salmon early in spring. The correlation in the historical occurrence of Chinook salmon in the three rivers suggests that they have been colonized as part of the same introduction event. Moreover, the temporal correlation with the development of ocean net pen production for this species in the Chiloé region suggests that the fish may originate from escapes from these enterprises. As in all previous reports of exotic Chinook salmon in Patagonia and New Zealand, populations in the three rivers have a combination of stream and ocean ecotypes. We discuss the importance of plasticity in critical life history characteristics for the success of Chinook salmon in Patagonia, compared to other species of anadromous salmonids.