A new species of green freshwater hydra (Cnidaria, Hydrozoa: Hydrida), Hydra sinensis, is described from Guangdong Province, China. The chief distinction between H. sinensis sp. nov. and three other green hydras (H. hadleyi, H. viridissima, and H. plagiodesmica) is in the holotrichous isorhizae. Hydra sinensis sp. nov. differs from H. plagiodesmica in the shape of the holotrichous isorhizae, and from H. viridissima and H. hadleyi in the tubule of the capsule of the holotrichous isorhizae. The capsule tubule coils two times in 86% and three times in 14% of holotrichous isorhizae (n=50) in H. sinensis sp. nov.; we observed no tubules coiling four times. In contrast, the capsule tubule coils three or four times in H. viridissima and H. hadleyi, and no tubules coiling two times have been reported. In addition, holotrichous isorhizae, which are mainly located around the hypostome, are sparse in the tentacles of H. sinensis sp. nov., whereas the majority of holotrichous isorhizae is located on the tentacles in most other hydras. A molecular phylogenetic analysis using the nuclear small subunit (18S) ribosomal RNA gene indicated a close relationship between H. sinensis and H. viridissima. Hydra viridissima did not group within a clade of four individuals of H. sinensis, indicating a possible sister-species relationship between the two species. Morphological characters in combination with the molecular phylogenetic evidence support Hydra sinensis as a new species.