A moisture absorbent acrylonitrile-based fiber configured from a polymer, said polymer being substantially free from any crosslinked structure via covalent bonds, is characterized in that: carboxyl groups are uniformly distributed all over the fiber; the content of the carboxyl groups in the fiber is 0.2-4.5 mmol/g; and, at 20°C × 65%RH, the saturated moisture-absorbing ratio of the fiber is 5 wt% or more and the swelling degree in water thereof is 10 times or less. This moisture absorbent acrylonitrile-based fiber requires no step of introducing a crosslinked structure and has sufficient fiber properties, which enables continuous production thereof. The moisture absorbent acrylonitrile-based fiber has high water absorbent properties and, moreover, shows no increase in redness during use. In addition, shrink properties, easy-decrimping properties, bulkiness and water repellency can be easily imparted to the fiber.