Abstract Volcanic eruptions can produce large magnetic field changes by thermomagnetic effects, especially when magma cools from high temperatures and acquires a permanent magnetisation from the Earth's magnetic field. After the 2000 eruption of Mt Usu, Japan, significant magnetic field changes were observed not only in the vicinity of the magmatic intrusion but also in an area some distance away that was unlikely to be at a temperature near the Curie Point. The magnetic changes in the latter area appear to be caused by thermal viscous remanent magnetisation (TVRM), in which the elevated subsurface temperatures have accelerated the acquisition of magnetisation parallel to the existing Earth's field in the ground material. The unusually large changes (up to 40 nT/yr) are probably because of underlying reversely magnetised Pleistocene rocks, which are being normally remagnetised by exposure to temperatures of over 200 °C. We made an order of magnitude estimate of the likely effects of TVRM in this case, based on some previous laboratory studies, which confirms that TVRM is a plausible mechanism for the observed magnetic changes. This paper provides probably the first field example in which a natural TVRM process associated with ongoing volcanic activity has been observed. This indicates that even when temperatures are well below the Curie Point, significant magnetic changes can occur on active volcanoes. In many cases, areas of recent volcanism are dominated by normal magnetisation and thus the TVRM only plays a minor role in volcanomagnetic changes. It will, however, be a significant effect when materials with reversed or randomly-oriented magnetisation are moderately reheated through volcanic activity. We emphasize that separation of thermal viscous magnetisation from thermal magnetisation will sometimes be necessary for properly interpreting magnetic changes around volcanoes, especially in relation to ongoing volcanic activities.