In the present study, a cheese whey agro-industrial byproduct was utilized as a natural feedstock for bioplastic production. The bioprocess consisted of a fermentative lactic acid production (step 1) and a following photofermentative poly(3-hydroxybutyrate) production (step 2). During step 1, the bacterium Lactobacillus sp. converted lactose (contained in cheese whey) into lactic acid. During step 2, the marine bacterium Rhodovulum sulfidophilum DSM-1374 converted lactic acid, contained in the cheese-whey fermented effluent (CWFE), in bioplastic. In this investigation, the CWFE produced during step 1 showed a lactic acid content of 29.5 ± 1.3 g/L. When, for feeding Rhodovulum sulfidophilum DSM-1374, was utilized CWFE diluted with water (50 %, v/v) the highest poly(3-hydroxybutyrate) content (67 ± 3.1 % of bacterial dry-biomass) was observed. Both bioprocess steps can significantly contribute to realize a circular bioeconomy able to cut down the costs of bioplastic production and reduce the environmental impact caused by the intensive human feed and food productions.