<p>Poly(lactic acid) (PLA) is widely used to replace fossil-fuel-based feedstocks, but its marine biodegradability is not sufficient to avoid concerns about plastic pollution in the ocean. To improve its biodegradability, PLA was melt-blended with 10–40 wt% poly(hydroxyl alkanoates) (PHA) and used to prepare films. The samples were wrapped in mesh nets and immersed in a marine tank to observe degradation over 45&#xa0;weeks. The marine degradation of the PLA/PHA blends was investigated based on its residual weight, molecular weight, visual appearance, chemical structure, and thermal and morphological properties. Notably, as the PHA content increased, the weight loss increased. The residual weight of the PLA60/PHA40 blend was only 30.6% of its initial weight. In all samples, marine hydrolysis reduced the molecular weight, increased the polydispersity index, and changed the chemical structure. Furthermore, the blended samples exhibited a reduced glass transition temperature and increased crystallinity. In terms of thermal stability, the onset temperature of decomposition of the PLA60/PHA40 blend decreased from 313.9 to 294.3&#xa0;°C. Moreover, the surfaces of the films became rough after marine decomposition, which worsened with increasing PHA content. These phenomena may be attributed to pores formed within the polymer matrix owing to the preferential decomposition of PHA in the PLA/PHA blends. These pores may have formed pathways for seawater and microbe penetration, thus exacerbating degradation. Consequently, a PHA content of 40 wt% in the binary blends is expected to promote degradability.</p> Graphical abstract <p></p> <p>These graphs show that the degradation of PLA/PHA blends under seawater conditions is promoted as the PHA content increases. These results support that PHA can accelerate marine degradation in blend materials</p>