This study investigates a sequential dual-modification strategy to enhance the functionality of potato starch by combining ultrasonication-assisted nanoprecipitation with carboxymethylation. The goal was to create carboxymethylated starch nanoparticles (CMSNPs) with improved surface charge, substitution efficiency, rheological behavior, and bioactive carrier potential. Starch was first downsized into starch nanoparticles (SNPs), yielding particles with an average size of about 70 nm and a narrow size distribution. Subsequent carboxymethylation produced CMSNPs with an average size of about 400 nm and introduced negatively charged carboxylate groups, confirmed by FTIR peaks near 1600 and 1410 cm⁻¹. The degree of substitution increased significantly to about 0.08, while zeta potential became more negative and pH-responsive. CMSNPs also showed improved viscosity retention in the presence of Ca²⁺ and promising encapsulation performance for positively charged bioactives, reaching up to 45% encapsulation for thiamine. These results suggest CMSNPs are a promising platform for food and nutraceutical delivery applications.