Liquid metal nanodroplets not only share similar metallic properties and nanoscale effect with solid metal nanoparticles, but also possess the additional uniqueness in nonvolatile fluidity and ambient sintering ability into continuous conductors. In most cases, liquid metal nanodroplets are encapsulated into ultrathin and fragile shells of oxides and amphiphile monolayers, and may be hindered from incorporating homogeneously into various composites through conventional processing methods. In this study, ring‐opening polymerization is found to be initiated by sonicating the liquid metal EGaIn in fluidic lactones. By this in situ polymerization, EGaIn nanodroplets are encapsulated into polylactone shells with tunable thickness, which can further be dried into a solid powder. Besides high chemical stability and dispersibility in organic solvents, the powder of the EGaIn capsules combines the exceptional properties of the EGaIn droplets (e.g., photothermal effect) and the polylactone shells (e.g., biocompatibility, biodegradability, and compatibility with different polymer matrixes), being capable of being introduced into thermoplastic composites through liquid casting and thermal‐ or photomolding for the notch‐insensitive tearing property, sintering‐induced electric conductivity, and photothermal effect. Thus, the EGaIn initiator of ring‐opening polymerization may start a pathway to produce stable andthermal/photomoldable powders of EGaIn capsules and their multifunctionalcomposites, applicable in biomedicines, soft electronics, and smart robots. Ring‐opening polymerization initiated by sonicating EGaIn in fluidic lactones enables self‐capsulation of EGaIn droplets into polylactone shells with tunable thickness. With thermal/photomoldability, biocompatibility, biodegradability, and high photothermal efficiency, powder of EGaIn capsules is ideal for multifunctional composites (e.g., with sintering ability, notch‐insensitive tearing property, electric conductivity, and photothermal effect), applicable in stretchable electronics, biomedicines, and smart materials.