The southernmost Okinawa Trough back-arc basin is an active and young basin formed just after the collision of the Philippine Sea Plate against the Eurasian continental margin. The back-arc extension occurs intensively because of the southward or southeastward migration of the southernmost Ryukyu Arc, or the roll-back of the Philippine Sea Plate. To better understand the active tectonics and volcanism of the southernmost Okinawa Trough, we have conducted deep-tow sub-bottom profiler and side-scan sonar surveys across the back-arc basin. Our results show that the volcanism of the southernmost Okinawa Trough is distributed in the southern half of the back-arc basin and occurs along some linear or branched zones roughly parallel to the trough axis. Volcanic seamounts are obviously located along the central depression of the basin and their sizes show lateral variation. On the other hand, the northern half of the southernmost Okinawa Trough back-arc basin has almost no volcanic activity, but contains more brittle normal faults. It is noted that gas plumes out of seafloor are generally associated with hydrothermal mounds or activities, instead of volcanic seamounts. We suggest that the more complete rifting of the southernmost Okinawa Trough back-arc is limited to the east of ~122o30′E. To the west of ~122o30’E, the back-arc extension could be still influenced by the inherited NE-SW trending structures of the continental crust created during the former Taiwan orogeny in this area.