Background and objective In a previous study, the authors produced, optimized, characterized, and purified the poly3-hydroxybutyric acid (PHB) using the locally isolated Bacillus flexus bacterial strain. The development of a biodegradable drug carrier as an efficient delivery system has received great interest over the past few decades. The objective of this study was to produce a nano-PHB carrier for curcumin to be more effective in tumor fighting. Materials and methods PHB was produced on the optimized medium by B. flexus strain. The nano-PHB form was produced using the nanoprecipitation technique. The size, shape, and characteristics of loaded curcumin nano-PHB particles were performed using zeta-potential, scanning electron microscopy, and Fourier transform infrared spectroscopy techniques. The antitumor effect of the nano-PHB loaded curcumin was performed using human breast adenocarcinoma MCF-7 cell line. Results and conclusion In this study, a naturally developed, biodegradable, and biocompatible nanosized carrier for curcumin-targeted delivery in breast-cancer cells with higher encapsulation efficiency (95.5%) was formulated. The size range of both free PHB and curcumin-loaded PHB was 237 and 260 nm respectively. The nanoparticles exhibited a spherical shape with no aggregation which is confirmed by electron microscopy, indicating a higher colloidal stability. The curcumin-loaded PHB nanocarrier showed a sustained drug release behavior. In-vitro anticancer assays showed the superiority of curcumin-loaded PHB nanocarrier over free curcumin for fighting breast cancer. These results show that the PHB biopolymer acts as an efficient carrier vehicle for the curcumin.