Large-scale green tide disasters have been occurring for more than 10 years in China, with serious ecological consequences. However, limited research has focused on epiphytes that feed on macroalgae. In this study, amphipods were found living on the surface of Ulva lactuca, a green-tide-forming alga, around Gouqi Island, China. Through morphological and molecular methods, the amphipods were identified as Ampithoe lacertosa Bate, 1858, which has not previously been recorded in China. Sequence information about cytochrome c oxidase subunit I (mitochondrial COI) gene from China was compared to sequences reported from 5 coastal countries around the Pacific region. Network diagrams, phylogenetic trees and other methods of analysis revealed evidence of genetically distinct subspecies in the northeastern and northwestern Pacific. We believe that research on herbivorous amphipods will provide preventative strategies and control measures against green tide algae. International cooperation among Asian and North American biologists within the Pacific region will lead to a better understanding of the family Ampithoidae in the future.