This study investigated the acute toxicity of cadmium (Cd), copper (Cu), lead (Pb), aluminium (Al), and zinc (Zn) to first-instar Polypedilum nubifer larvae and examined the influence of ultrafine bubble (UFB) water on metal toxicity. Larvae were exposed to a range of metal concentrations in control water (dechlorinated tap water) and in UFB treated water, while physicochemical parameters such as particle size, zeta potential, dissolved oxygen, pH, and temperature were characterized. Acute toxicity was assessed over 48 hours following OECD Guideline 235, and LC50 values were estimated using probit analysis. Pb showed the highest acute toxicity in control water (LC50 = 0.84 mg/L), followed by Cu (LC50 = 0.92 mg/L), whereas Cd, Al, and Zn showed low toxicity, with LC50 values exceeding the maximum tested concentrations. Exposure in UFB water increased survival rates of larvae exposed to Pb and Cu, raising LC50 values to 1.06 mg/L and 1.33 mg/L, respectively. Physicochemical characterization indicated the presence of nanoscale bubbles with an average diameter of 172 nm and negative zeta potential of -12 mV, which likely reduced the bioavailability of dissolved metal ions. These findings demonstrate that UFB water may decrease the acute toxicity of Pb and Cu to P. nubifer, potentially by reducing free metal ions and mitigating oxidative stress. Overall, the results establish first-instar P. nubifer as a sensitive bioindicator of metal toxicity and demonstrate that UFB technology can modify metal bioavailability in freshwater ecosystems.