<p>In this paper, we studied the effects of poly[di(ethylene glycol) adipate] (A) on poly(lactic acid)/polyhydroxybutyrate (PLA/PHB) blends to develop a suitable material for biodegradable single-use packaging. The blends were prepared by melt-blending followed by compression molding. This natural additive was selected because polyadipates are effective plasticizers for PHB, improving flexibility and processability by lowering the glass transition temperature and the melt fluidity. However, no studies have investigated its effect on blends of PHB and PLA. Additionally, polymeric plasticizers offer the advantages of low migration and high thermal stability, which prevent their loss during standard industrial polymer production processes, compared to low molecular weight additives. Therefore, the impact of these plasticizers on the morphology, as well as on the thermal, mechanical, and optical properties of these blends, was investigated. The disintegrability under composting conditions was also studied to assess the sustainability of this alternative to traditional polymers. It was found that the glass transition temperature (<i>T</i>_<i>g</i>) and the melting temperature (<i>T</i>_<i>m</i>) of PLA and PHB decreased as the amount of A increased. Additionally, the presence of A tended to increase the crystallinity of the sample, a result supported by X-ray diffraction. The effect of the plasticizer addition on the mechanical properties of the films was consistent with its effect on <i>T</i>_<i>g</i> and crystallinity. Thus, A demonstrated an excellent plasticizing effect for PLA, PHB, and their blends by reducing strong hydrogen bonds and secondary interactions between polymer chains.</p>