With the increasing demand for instant real-time ultrasound (US) imaging of a specific organ, target-specific and long-circulating ultrasound contrast agents are of special interest. A new species of echogenic hyaluronic acid nanoparticles is presented as an ultralong-acting, liver-specific, US contrast agent that is distinct from conventional gas-filled microbubbles. Using an oil-in-water (O/W) emulsification method, bioinert and hydrophobic perfluoropentane (PFP) is encapsulated as an ultrasound gas precursor into hyaluronic acid nanoparticles (HANPs) using hydrophobic interactions. HANPs are formulated by self-assembly, with amphiphilic hyaluronic acid-5-cholanic acid (HA-CA) conjugating in aqueous conditions. The resulting echogenic PFP-encapsulated HANPs (Echo-NPs) show solid nanostructures, differentiated from core-empty conventional microbubbles, and exhibiting outstanding physical properties as an ultrasound contrast agent. They are more stable and robust echogenic solid bodies with an in vivo favorable hydrodynamic size and because PFPs vaporize gradually, their expansion process is very slow in body conditions. After several systemic circulations, echo-NPs generated intense and ultralong echo signals for US imaging at the target site. The echogenic properties of Echo-NPs show a significantly increased half-life and echo persistence, compared with conventional microbubbles. The results clearly show that echo-NPs outperform conventional microbubbles in terms of both physical and echogenic in vitro and in vivo properties.