This paper presents a parametric study of Vertical-Axis Turbines (VAT) equipped with chordwise-flexible blades. The objective of this study is to assess the possibility of increasing the efficiency of VATs by allowing passive foil deformations that take advantage of the periodically changing hydrodynamic conditions inherent to VATs. The study is carried out with numerical simulations based on a partitioned Fluid–Structure Interaction (FSI) code implemented within OpenFOAM. Two-dimensional URANS numerical simulations are carried out at a Reynolds number of 10⁷ based on the turbine diameter, which is representative of hydrokinetic applications. A single-blade rotor equipped with a chordwise-flexible NACA 0015 profile is considered. The flexibility is located in the rear part of the foil, between the attach point and the trailing edge. The tip-speed ratio range extends from 2 to 8 and the solidity is set to 0.286. The characteristics of the blade are specified with a dimensionless flexibility parameter and a so-called pressure-to-inertia ratio. The pressure-to-inertia values investigated are larger than one and are representative of hydrokinetic applications in water. The flexibility is varied from zero (rigid blade) to large values involving large deformations. The FSI effects related to these two parameters are investigated and the mechanisms that allow efficiency improvements are presented. It is shown that flexibility introduces a stall mitigation mechanism that helps in improving the efficiency of low tip-speed ratio configurations. Flexibility also reduces blades’ drag in the downstream portion of the cycle when the turbine operates at high tip-speed ratios. More specifically, relative efficiency improvement of about 12% to 15% are observed with moderate flexibilities at tip-speed ratios of 2 (lower than optimal) and 6 (higher than optimal). However, at the optimal tip-speed ratio, flexibility strictly reduces the efficiency, although this reduction remains small for moderate flexibilities. Overall, results also show that blade flexibility tends to extend the tip-speed ratio range of high-efficiency configurations, which may provide interesting advantages when the turbine operates in changing flow conditions.