Poly(3‐hydroxybutyrate) (PHB), is one important biopolymer and a promising alternative to petroleum‐based plastics. In this article, formulations of PHB and triethyl citrate (TEC) as plasticizer were prepared by melt extrusion. The effect of TEC on the mechanical, thermal, and morphological properties of PHB was investigated by tensile tests, impact resistance, dynamic‐mechanical analysis, differential scanning calorimetry, polarized optical microscopy, and small‐ and wide‐angle X‐ray scattering. TEC acted as an efficient plasticizer for PHB, imparting gradual changes in the properties as the mass fraction of TEC increased. A reduction in the elastic modulus, an increase in the intensity of β relaxation indicated a higher capacity of mechanical energy dissipation for the formulations containing higher mass fractions of TEC. TEC reduced its glass transition and melting temperatures, contributing to the increase of the processing window of the temperature and minimizing thermal degradation of PHB. TEC had a strong influence on the kinetics of crystallization, the morphology of the spherulites, and the crystalline structural parameters, such as long period, crystalline lamella, and interlamellar amorphous region thicknesses. Our study clarifies how the morphology of the PHB crystalline phase evolves in the presence of the plasticizer and with the time of crystallization. PHB is a stiff and brittle polymer that degrades at temperatures close to the melting. The addition of variable amount of the plasticizer triethyl citrate to PHB increased the flexibility and the impact resistance, improved the processability and reduced the thermal degradation. The results showed that triethyl citrate is among the most efficient plasticizers for PHB reported in the literature.