Diatoms are unicellular photosynthetic algae which constitute the most important and dominant fraction of marine phytoplankton. They are primary producers forming the basic link in the marine pelagic food chain. They also help mitigate the global warming effect through sequestering the green house gases. The planktonic diatoms of the N. Arabian Sea bordering Pakistan, which is characterized by very high primary productivity due to monsoonal upwelling, has long been neglected by marine biologists. The present study deals with taxonomy and ecology of centric diatoms of the area. As many as 79 species belonging to 23 genera are described along with their spatial and temporal distributions. Chaetoceros and Rhizosolenia were the most diverse genera among all with 17 species belonging to each. Several species were first records from the area and a variety Planktoniella blanda var. bilobata even new to the knowledge of science. A peak in species diversity was observed in February during the NE-monsoon season. This study fills an important lacuna in the knowledge of marine diatoms from the world oceans and will appeal not only to marine biologists but also environmentalists.