Songkhla Lagoon in southern Thailand is a large lagoon with brackish water where the mysid Nanomysis siamensis inhabit. N . siamensis has long been the dominant mysid species in this lagoon. However, nothing is known about the biology and ecology of N . siamensis . Here for the first time, we provide information about its demographic structure and reproductive biology in the Songkhla Lagoon system. We conducted monthly sampling from June 2017 to June 2018 at five stations in Thale Sap and seven stations in Thale Sap Songkhla. Individuals were classified into six categories. The monthly mean density ranged 8.44–807.33 inds./100 m 2 in Thale Sap and 8.81–434.44 inds./100 m 2 in Thale Sap Songkhla. The peak occurrence of this species was in May 2018 in Thale Sap. The population composition was as follows: juvenile 10.13％, immature males 13.45％, immature females 10.54％, mature males 26.19％, mature females 21.63％ and ovigerous females 18.05％. The proportion of ovigerous females was statistically significant and brooding females in the nauplioid stage were more dominant than embryonic phase and post-nauplioid stages. Brood size varied from 1 to 17, the embryo diameter ranging from 0.2 to 0.4 mm and the lengths of the larvae ranged from 0.5 to 1.0 mm. There was a positive correlation between brood size and brood length with ovigerous female body length. Overall, mature males were predominant over the five population composition types, and reproduction in N . siamensis was observed every month.