Protokeelia is described as a new and rare pennate diatom genus in China. Its valve includes two parts: ventral valve and dorsal valve. The ventral valve is more or less convex or concave. The raphe runs in a distinct elevated and thickened siliceous ridge. In the internal valve, beneath the raphe are fibulae or fibulae clusters which connect the ventral and dorsal valves. One species, P. Spinifera Round & Basson, is observed in the water samples from a mangrove forest in Sai Kung, Hong Kong, China. Its main morphological features are described in this paper along with electron microscope (EM) photos. Morphological comparisons are also made among several allied species. In addition, a newly recorded species for China, Rhopalodia iriomotensis Kobayasi, Nagumo & Tanaka, is discussed here. Because several key features observed in this species resemble those of some Pro-tokeelia species, we suggest that it may actually represent a Protokeelia species.