Understanding precipitation formation at lanthanum hydroxide (La(OH)3) nanoparticle-solution interfaces plays a crucial role in catalysis, adsorption, and electrochemical energy storage applications. Liquid-phase transmission electron microscopy enables powerful visualization with high resolution. However, direct atomic-scale imaging of the interfacial metal (hydro)oxide nanostructure in solutions has been a major challenge due to their beam-driven dissolution. Combining focused ion beam and aberration-corrected high-angle annular dark-field scanning transmission electron microscopy, we present an atomic-scale study of precipitation formation at La(OH)3 nanoparticle interfaces after reaction with phosphate. The structure transformation is observed to occur at high- and low-crystalline La(OH)3 nanoparticle surfaces. Low-crystalline La(OH)3 mostly transformed and high-crystalline ones partly converted to LaPO4 precipitations on the outer surface. The long-term structure evolution shows the low transformation of high-crystalline La(OH)3 nanoparticles to LaPO4 precipitation. Because precipitation at solid-solution interfaces is common in nature and industry, these results could provide valuable references for their atomic-scale observation.