While the ocean offers significant potential for harnessing hydrokinetic energy, extracting this energy in regions with low flow velocity remains a challenge. The objective of this study is to investigate the improvement in the power output performance of a drag-based hydrokinetic turbine (HKT) operating in a low flow velocity of 1 m/s by adding a diffuser in the turbine design. Three-dimensional numerical simulations are carried out by solving the unsteady Reynolds-Averaged Navier-Stokes (URANS) equations with the renormalized group (RNG) k-ε turbulence model. Parametric studies on the effects of diffuser's shape (flat and curved) and length (DL = 0.5 m, 0.65 m and 0.8 m) are carried out by examining the corresponding power output performance of the HKT. Generally, adding the diffuser on the HKT improves the power output performance of the HKT as compared to the setup without the diffuser. It is also noted that the curved diffuser gives better power output performance as compared to the flat one. In addition, the diffuser can provide a shielding effect to the turbine blade to eliminate severe turbulences at the downstream region. This shielding effect becomes more pronounced with the increasing diffuser length. As a result, this configuration minimizes energy losses and enhances overall turbine efficiency. © 2024 Elsevier Ltd