Marcus Island LORAN-C transmitter, Radio navigation transmitter in Minami-Tori-shima, Japan.

Marcus Island LORAN-C transmitter stood on Minami-Tori-shima in Ogasawara, Tokyo, at 24°17′8″N 153°58′54″E. Its guyed mast radiator served as the X-Ray secondary station of the North West Pacific LORAN-C chain GRI 8930, transmitting at 4000 kilowatts until 1993, more than the most powerful broadcasting stations ever used. The mast was rebuilt in 1985/86 and again in 2000, both times at 213.4 metres.

The facility began operating in 1963 under United States Coast Guard control. It transferred to Japanese Coast Guard management in 1993, marking the end of an era as satellite navigation soon replaced older radio methods.

The transmitter served as a navigation landmark for ships from many nations crossing the Pacific. It represented a practical partnership between American and Japanese mariners who relied on the same technology for safe passage.

The site sits on a remote island and is normally not easily accessible to visitors. Information about the station today is found mainly in maritime archives and historical records.

The original transmission mast stood at over 400 meters tall, making it one of the world's tallest structures of its era. Few people realize this engineering feat was accomplished on one of Japan's most isolated islands.