The seaweed-derived bioactive products are a storehouse of healthy attributes. Researchers across the globe are waking up to tap these unexploited marine sources to develop novel therapeutics. Among the three main divisions of marine macro algae (Chlorophyta, Phaeophyta and Rhodophyta), the marine green algae (Chlorophyta) especially, Codium genus has been of public and scientific concern from the last two decades, because some of these are invasive, bloom-forming nature. Among them, Codium fragile is the most invasive seaweed in the world. It is believed to be native to Japan and was then unintentionally spread around the world. Codium fragile, sponge seaweed and it is well known in the name of “OYSTER THIEF”, because it usually attach to oysters and float away, carrying the animals with it. This was the inspiration for the common name “oyster thief”. It constitutes useful raw materials for the development of diets or ingredients for human and animal nutrition and has different biological activities with a great potential in pharmaceutical applications. It biosynthesizes sulfated polysaccharides with very distinct structural features. So focus on marine green alga Oyster thief (Codium fragile) with an overview of the recent progress of its structurally diverse of bioactive materials/compounds and biological activities. © 2016, Sphinx Knowledge House. All rights reserved.