Abstract The mysid Rhopalophthalmus hastatus Hanamura, Murano and Man, 2011 was collected from the shallow zone of the Thale Sap Songkhla, Songkhla Lagoon, southern Thailand on 18 November 2018. Specimens are described and illustrated using material collected by hand with a Riley push net. Rhopalophthalmus hastatus is characterized by the presence of one large spine, one mesial, two small spines on the antennal sympod, antennal scale slightly overreaching the end of the antennular peduncle, carpopropodus of the fourth to seventh thoracic endopods composed of six articles, two red chromatophores on the telson and lateral margin of the apical spine forming sharp spine-like process. This species was found in the shallow zone of the Songkhla Lagoon with brackish water and muddy substratum.