In North America, Salmo salar (Atlantic Salmon) populations evolved in the absence of Salmo trutta (Brown Trout) and use estuaries more extensively than in Europe. European Brown Trout were introduced to Newfoundland in the 1880s and are spreading along its coast. Most of the colonized watersheds include distinct estuaries. Unlike for river habitats, knowledge of estuary use by Brown Trout outside of their native range is very limited. We investigated seasonal estuary use by Atlantic Salmon and Brown Trout of different sizes in eastern Newfoundland. We observed parr-sized Brown Trout in June and July, whereas Atlantic Salmon parr were present from April to August. Smolts of both species were most prevalent in spring, but we found them throughout the year. Large Brown Trout were present in the estuary over much of the year. Brown Trout are a freshwater invasive species in much of their introduced range and their year-round estuarine presence in our system raises concerns for native salmonids, particularly Atlantic Salmon on Newfoundland’s south coast, which have been identified as at-risk and evolved in the absence of these invaders.