<p>Hydrogels with photothermal properties were developed using polyaniline nanoparticles for potential applications in photothermal therapy and cosmetics. Polyaniline, a cost-effective, photothermal conjugated polymer, was modified with octyl side chains to enable to produce an aqueous nanoparticle dispersion by assembling with an amphiphile via a film dispersion process. In comparison to pristine polyaniline dissolved in water, polyaniline nanoparticles presented improved photostability and bathochromic shift in their absorption spectra due to conjugated backbones protected in nanoparticles, which shield the conjugated backbones from the polar aqueous environment. Alginate hydrogels were successfully fabricated by mixing the water dispersed conjugated polymer nanoparticles with a sodium alginate solution, followed by calcium ion crosslinking. Compared with sodium alginate hydrogels without nanoparticles, the obtained hydrogels showed significantly improved mechanical and photothermal properties due to the contributions of polyaniline nanoparticles as both a nanofiller and a photothermal agent. The enhanced stability and sustained photothermal performance are critical features for applications requiring prolonged activity under oxidative stress, such as photothermal-mediated therapies.</p> Graphic abstract <p>Alginate hydrogels composited with polyaniline nanoparticles presented photothermal properties with enhanced mechanical properties.</p> <p></p>