With the discussion of sustainability and the increased environmental awareness, the biopolymer poly‐β‐hydroxybutyrate (PHB) has gained more and more attention in recent years. It derives from renewable sources and can degrade in different environments to carbon dioxide and water. A hindrance for its potential application in plastics parts is its brittleness, deficiency in thermal stability and low crystallization rate. To overcome these problems chemical modification of PHB by reactive extrusion was carried out in a solvent‐free one‐step process by means of a twin screw extruder. Thermal and rheological characterization confirmed the branching and partial crosslinking of PHB by the organic peroxide tert.‐butylperoxybenzoate as well as by the reaction assisted multi‐functional coagents triallyl cyanurate and divinylbenzene. The crystallization temperature of the modified PHB could be significantly increased, up to 16 K, accompanied with an enhanced crystallization rate. Rheological studies show a distinct increase in the melt viscosity and the elastic properties dependent on the modifier content. The addition of plasticizers like rapeseed oil or polyethylene glycols (PEG 1500 and PEG 4000), respectively, lead to an increase in melt extensibility and melt viscosity. In case of PHB modified with PEG 1500, degradation reactions could be substantially retarded. POLYM. ENG. SCI., 59:1057–1064, 2019. © 2019 Society of Plastics Engineers