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Mytilopsis sallei is a small bivalve that has invaded Asian waters, including southeast Asia. This shellfish has become a pest in coastal and estuary waters in Southeast Asia. This study is the first report on the presence and structure of the M. sallei population in Indonesia, precisely in the coastal lakes of Medan City, North Sumatra Province. This research was conducted in Lake Siombak, Medan City, North Sumatra, which is one of the tropical tidal lakes in Indonesia. Mytilopsis sallei shellfish samples were taken for a year from September 2018--August 2019. Mytilopsis sallei were obtained using a Petersen Grab with a mouth opening of 30 x 30 cm. The measurement of M. sallei consisted of the shell length, shell width, shell thickness, and body weight. The analysis of the population structure parameters with ELEFAN I was accommodated in the FiSAT II software. Spatially, M. sallei highest density in Siombak Lake is at station 5 with 24,100 and 23,948 ind.m-2 and the lowest at station 7 is only 967 ind.m-2. Monthly, M. sallei hase the highest density in the dry season (Feb--Jul) compared to the wet season (Aug--Jan). The density of M. sallei in the dry season reaches 1213--13,187 ind.m-2 and in wet seasons reaches 369-3,029 ind.m-2. The Mytilopsis sallei in Siombak Lake has a maximum length of 42.14 mm and the smallest size is 3.00 mm. The growth model of M. sallei in Lake Siombak is Lt = 42.14 (1-e-1.1(t-0.1598)). Mytilopsis sallei recruitment took place in January (fifth month). The instantaneous mortality rate (Z) of M. sallei mussels was 5.32 with an exploitation rate (E) of 0.66. [ABSTRACT FROM AUTHOR]