A new tongue sole, Cynoglossus nanhaiensis, described from 21 specimens (101.0–133.7 mm SL) collected in coastal waters of the South China Sea, is distinguished from congeners by the following combination of characters: 3 ocular-side lateral lines; no blind-side lateral lines; 2 ocular-side nostrils; 8 caudal-fin rays; 49–51 total vertebrae; 64–73 scales in the midlateral line; 11–12 scales in diagonal series between midlateral and dorsal lateral lines; 99–108 dorsal-fin rays; 77–82 anal-fin rays; pores of lateral-line scales tubular, with conspicuously black tips posteriorly; and with light- to medium-brown ocular-side background coloration overlain by numerous, small, irregular, dark blotches forming a series of narrow, interrupted, longitudinal stripes, with many specimens also featuring two large, brown spots on their caudal region. Among congeners, C. nanhaiensis is most similar to C. maccullochi Norman, 1926, described from specimens taken off Queensland, Australia, but differs in its ocular-side pigmentation (conspicuous longitudinal series of irregular blotches and caudal spots absent in C. maccullochi) and body depth. Features of C. nanhaiensis are also reminiscent of those reported for C. dollfusi (Chabanaud, 1931), a poorly-known nominal species from the Red Sea, and C. itinus (Snyder, 1909), a better-known species occurring in marine waters off Japan and Southeast Asia. Cynoglossus nanhaiensis differs from both species in having two (vs. one) ocular-side nostrils. Cynoglossus itinus also lacks the series of conspicuous, darkly-pigmented, interrupted, longitudinal stripes on the ocular side that are characteristic of C. nanhaiensis. Cynoglossus nanhaiensis is further distinguished from C. dollfusi in having only a single pelvic fin (vs. two in C. dollfusi). Cynoglossus nanhaiensis occurs in coastal waters of the South China Sea off southern China and Viet Nam.