Methylocystis parvus OBBP accumulates polyhydroxybutyrate (PHB) using methane as the sole carbon and energy source. In this work, the feasibility of producing (R)-3-hydroxybutyric acid (R3HBA) via intracellularly accumulated PHB through depolymerization (in-vivo) was investigated. Results showed that a PHB to R3HBA conversion of 77.2 ± 0.9% (R3HBA titer of 0.153 ± 0.002 g L-1) can be attained in a mineral medium containing 1 g L-1 KNO3 at 30 °C with shaking at 200 rpm and a constant pH of 11 for 72 h. Nitrogen deprivation and neutral or acidic pHs strongly reduced the excreted R3HBA concentration. Reduced oxygen availability negatively affected the R3HBA yield, which decreased to 73.6 ± 4.9% (titer of 0.139 ± 0.01 g L-1) under microaerobic conditions. Likewise, the presence of increasing concentrations of R3HBA in the medium before the onset of PHB depolymerization reduced the initial R3HBA release rate and R3HBA yield.