Poly[(R)-3-hydroxybutyrate] (PHB) is a bacterial storage polyester that is genuinely biodegradable, but its brittleness hampers its wide applicability as a biodegradable plastic. Most attempts to improve the mechanical properties of PHB have focused on incorporating comonomers, but at the expense of production costs. Strikingly, it appeared possible to toughen PHB homopolymer using a simple annealing treatment. The present study attributes this remarkable phenomenon to a change in the lamellar morphology, which reduces the amorphous-crystalline interface area and thus the constraint imposed on the amorphous chains by the crystals. This elevates the relaxation strength of the amorphous regions, giving rise to the favourable fracture behaviour.