Since the 1950s seismic activity associated with submarine volcanic eruptions has been detected throughout the Pacific Ocean basin using hydroacoustic methods. However, narrowband harmonic tremor with multiple overtones typically associated with subaerial volcanic eruptions has not commonly been observed. This paper presents a series of recently recorded hydroacoustic signals that originated in the Volcano Islands arc south of Japan, a region that frequently experiences submarine eruptions. The signals are characterized by a narrowband, long-duration, high-amplitude fundamental centered at 10 Hz with three harmonics at 20, 30, and 40 Hz and are consistent with harmonic tremor signals observed using traditional seismic methods at active subaerial volcanoes throughout the world. These hydroacoustic signals were detected 21 different times over the period from April 1998 through December 1999, typically lasting from several days to several weeks. After a 6-month hiatus, the signals were detected 26 more times from June 2000 through August 2001. The signals are qualitatively interpreted as a result of resonance of a magma-gas mixture within a large chamber/conduit near the surface of a shallow (<1 km) submarine volcanic edifice, allowing for efficient release of acoustic energy into the ocean sound channel. During periods of peak energy radiation the tremor could be clearly detected on hydrophones in the western Aleutians, Hawaii, west coast of the United States, and eastern equatorial Pacific and was detected by the French Polynesian seismic network, covering a maximum distance of ~14,000 km.