We have developed a new hydrogel hemostat composed of hyaluronan (HA) conjugated with inorganic polyphosphate (PolyP). A hemostatic hydrogel, HAX-PolyP, was formed rapidly by mixing aldehyde-modified HA and hydrazide-modified HA conjugated with PolyP (HA-PolyP). Although the gelation rate decreased with increasing PolyP content, the gelation time was below 5 min. In addition, the hydrogel swelling volume decreased with increasing PolyP content, but the degradation rate did not depend on PolyP content and the hydrogel underwent complete degradation through hydrolysis over 3 weeks in phosphate buffered saline. HAX-PolyP showed similar biocompatibility with the HA hydrogel without PolyP conjugation in vitro and in vivo. Intraperitoneal administration of HAX-PolyP did not induce any adhesion in the peritoneum and clot formation in the lungs. Finally, HA-PolyP accelerated the coagulation rate of human plasma ex vivo, and HAX-PolyP showed as strong a hemostatic effect as fibrin glue in a mouse liver bleeding model in vivo.