The seasonal movement patterns of adult smallmouth bass (Micropterus dolomieu) were examined in the Lake Washington Ship Canal (LWSC), a long, narrow water body that connects Lake Washington with Puget Sound. Seventy-two smallmouth bass were captured, implanted with acoustic tags, and released back into the LWSC where they were captured. Smallmouth bass displayed a strong seasonal migration pattern between the LWSC and Lake Washington. Out of 57 smallmouth bass tracked for seasonal movement information, 82% migrated from the LWSC to Lake Washington sometime between June and October. Departure from the LWSC may be related to a combination of factors, such as cessation of spawning activity, reduced water quality conditions, and reduced prey availability. Smallmouth bass < 350 mm FL were more likely to overwinter in the LWSC than larger fish and if they did migrate to Lake Washington, they migrated later in August–October. After overwintering in Lake Washington, smallmouth bass moved back into the LWSC between early-March and mid-April. Smallmouth bass returned to the LWSC at the beginning of the spring warming phase, a behavior likely related to spawning activity. Smallmouth bass often showed some degree of site fidelity between years for both spring/summer locations in the LWSC and summer/fall/winter locations in Lake Washington. Because smallmouth bass often have a high degree of spawning site fidelity, there can be different populations within the same water body. Our results provide some preliminary evidence that LWSC smallmouth bass are reproductively isolated from smallmouth bass that inhabit Lake Washington year-round.