The seaweeds and seagrasses growing in the Hakata Bay were bimonthly investigated at the six localities (Karadomari, Imadzu, Myoken-Misaki, Makinohana, southern Shikanoshima and northern Shikanoshima) from April 2006 to February 2007. Totally, 112 seaweeds (14 Chlorophyceae, 37 Phaeophyceae and 61 Rhodophyceae) and 2 seagrasses were found. The number of species found at the localities was greatly reduced toward the inside of the Bay. The reduction was conspicuous in Phaeophyceae and Rhodophyceae, particularly in large Phaeophyceae (Laminariaceae and Sargassaceae). This seemed to reflect the higher degree of eutrophication in the inner part of the bay. A comparison was made between the result in this study and that obtained just ten years ago. Almost no difference was found in the number of seaweed and seagrass species at each locality, and as a whole a similar biodiversity was recognized. We suppose that the result corresponds with the water quality that remained unchanged during these ten years.