Polyhydroxyalkanoates (PHAs) belong to a class of polyesters of natural origin obtained by microbiological synthesis. Despite good mechanical properties, biodegradation, and biocompatibility, PHAs are hydrophobic biopolymers, so the introduction of hydrophilic materials plays an important role, allowing them to influence the properties of the resulting microparticles. The goal of this work was to obtain microparticles from poly-3-hydroxybutyrate (P3HB) in a composition with sodium alginate loaded with a model antibacterial drug (imipinem). The characteristics of the prepared microparticles were studied using SEM, a particle size analyser (Zetasizer Nano ZS), and a spectrophotometer (Cary 60). Microparticles were obtained via solvent evaporation from triple emulsions. It has been established that the addition of sodium alginate affects the properties of microparticles: it increases the size and reduces the surface charge. It was shown that the efficiency of encapsulating imipinem into composite microparticles was almost three times higher compared to P3HB microparticles. In a model medium, prolonged release of the antibacterial drug was demonstrated over 28 days. The study of the biocompatability of microparticles in the culture of NIH 3T3 cells showed no negative effect. Using model Gram-positive and Gram-negative microorganisms as an example, it was shown that microparticles loaded with antibiotic showed antibacterial activity. Thus, it has been shown that the addition of sodium alginate has a positive effect on the properties of P3HB microparticles; namely, it allows increasing the efficiency of drug encapsulation, enhances the release of the drug from the microparticles, does not cause toxic reactions in cell culture, and also preserves the activity of the medicinal drug, which allows us to conclude that these composite microcarriers are promising for the development of long-acting dosage forms. This study was funded by the State Assignment of the Ministry of Science and Higher Education of the RF (project No. FWES-2021-0025).