The invention provides a process for producing 3-hydroxybutyric acid or a salt thereof. The process includes (1) culturing one or more halophilic bacteria belonging to the genus Halomonas under aerobic conditions in a medium containing an inorganic salt and one or more organic carbon sources; (2) changing the culture conditions from aerobic culture to microaerobic culture, and culturing bacterial cells of the halophilic bacteria to produce 3-hydroxybutyric acid or a salt thereof in a culture medium; and (3) collecting the 3-hydroxybutyric acid or the salt thereof from the culture medium.