A new species of the genus Sonagara Moore, 1882 (Lepidoptera: Thyrididae: Striglinae), S. bifurcatis sp. nov., is described from southern China and northern Vietnam. The new species is similar to S. strigipennis Moore in external morphology, but can be easily distinguished by the forewing having a brownish black line stretched from the apical angle to the outer side of inner margin, and the male genitalia with gnathos having peg-like teeth and with a short and broad valva. Adults and male genitalia are illustrated. The holotype is deposited in the Department of Entomology, South China Agricultural University, Guangzhou, China.