The hydrodynamic analysis of a horizontal axis hydrokinetic turbine is carried out, using a third-generation diffuser. The turbine constituted of three blades and a radius of 0.75 m, using the hydrodynamic profile NREL S822 for the design of both the blades and the housing of the 3rd generation diffuser. Both models were meshed and subsequently hydrodynamically simulated using the ANSYS CFX 18.2® program in a transient state. For analysis, it was used the k-e turbulence model, with a water input speed of 1.5 m/s and a variation of the angular velocity from 0 to 160 RPM, with steps of 10 RPM. As a result, the power coefficient (Cp) of the turbine was obtained with and without a diffuser, in addition to the velocity profile of each model. The maximum power coefficient (Cpmax) reached by the turbine with and without diffuser is 0.487 at a TSR of 95, and 0.285 at a TSR of 100, respectively, equivalent to an increase of 41.5% of Cp with respect to the turbine without diffuser, and 82.1% with relation to the Betz limit. Speeds are reached between 1.5 to 2.1 m/s and 1.5 to 1.8 m/s, upstream of the turbine with and without diffuser, respectively, and from 2.3 to 2.6 m/s and 1.8 to 2.1 m/s downstream of it, in the same order