Biochar is an excellent, but less-used candidate to serve as an alternative filler in poly(3-hydroxybutyrate) (PHB)-based composites. Increasing amounts of biochar between 20 and 50 wt% were incorporated in PHB/char and PHB/thermoplastic starch (TPS)/char composites and its effects on the microstructure, crystallization and thermal properties were investigated. PHB shows a significant reduction in molecular weight after processing and the increasing amounts of biochar decreases this even stronger. From thermogravimetric analysis, it was clear that the onset degradation temperature of the PHB/char composites (255 °C) is only slightly influenced by the biochar particles up to 40 wt%. Contrastingly, this temperature reduces to 245 °C when 50 wt% of biochar is added. Additional data confirm that morphology and crystallization kinetics are enhanced up to 40 wt% of biochar, while even higher percentages of filler clearly have an opposite effect. Finally, this work reveals the ability of TPS to work as an excellent intermediator between biochar and PHB at biochar concentrations up to 20 wt%, where degradation and resulting reduction in molecular weight remains limited as compared to that of the PHB/char sample. Furthermore, like biochar, TPS acts as a nucleation agent in the composites and removes the influence of the biochar on the thermal stability of the composites.