Our assessment of Rotifera biodiversity of the floodplain wetlands of the Majuli River Island of the Brahmaputra basin in Assam state, northeast India (NEI) reveals a total of 175 species belonging to 39 genera and 19 families. The rich and diverse assemblages of the phylum categorize the Majuli as one of the megadiverse Rotifera region of India. The observed biodiversity of  Rotifera is hypothesized to be associated with the habitat diversity of the Majuli wetlands, including its varied aquatic macrophytes, the location of the study areas in the ‘Indo-Burmese biodiversity hotspot’ and the ‘Rotiferologist effect’, and merits conservation interest in light of the extinction threat to this alluvial floodplain. We record one rotifer species as new to the Indian sub-region and NEI, and 29 species as new records from the Majuli. We also consider various newly recorded species to be of global and regional biogeography interest, with some hypothesized to have a biogeographic role linked to the ‘Assam gateway’. High richness of Lecane > Lepadella ≥ Trichocerca, the richness of Testudinella, and the paucity of Brachionus species are noteworthy features. Reports of 175 species from floodplain lakes and 148 species from small wetlands indicate biodiverse rotifer assemblages in these two categories of wetlands and present a useful contribution to Rotifera ecological diversity of the Indian and the tropical floodplains.