The 14–16 cal ka caldera-forming eruption of Nigorikawa volcano, Southwest Hokkaido, Japan, simultaneously ejected anomalously Ba-rich gray pumice (high-Ba gray pumice), low-Ba gray pumice, and white pumice. We investigated the petrological characteristics of each type of pumice to reveal the genesis of the high-Ba gray pumice. Although the glass compositions and mineral assemblages in the coexisting high- and low-Ba gray pumices are the same, the high-Ba gray pumice typically contains fine-grained, argillaceous material in the vesicles. Chemical analyses and X-ray diffraction patterns indicate that the fine-grained fill is composed of Fe-allophane and kaolinite with high Ba contents formed by hydrothermal alteration (dilute acid–neutral). The freshness of the volcanic glass hosting the filled vesicles and the stratigraphic variation in pumice componentry suggest that the fills were precipitated by trapped hydrothermal water in a relatively rapid process (e.g., the recycling of juvenile pumice) during the caldera-forming eruption. Similar hydrothermal alteration and recycling generating high-Ba gray pumice in the vent might occur frequently during explosive eruptions, and the possibility of in situ alteration should be carefully evaluated, even for young (Holocene) volcanoes.