Mahidolia mystacina (the flagfin prawn goby, flagfin shrimpgoby or smiling goby) is a species of goby native to the Indian Ocean and the Pacific Ocean from Delagoa Bay, Mozambique to the Society Islands and from southern Japan to Samoa and northern Australia. This species occurs in marine and brackish waters, being found in coastal bays, estuaries and reef bases where the bottom is silty or muddy at depths of from 5 to 25 metres (16 to 82 ft). This species is a commensal with a species of alpheid shrimp, using its burrow as its home. This species can reach a length of 8 centimetres (3.1 in) TL. This species can also be found in the aquarium trade. It is currently the only known member of its genus.