A fuel rod support insert for a nuclear fuel assembly spacer grid, a spacer grid and a nuclear fuel assembly are provided. The support insert is for a nuclear fuel assembly spacer grid comprising interlaced straps defining a lattice of cells for receiving fuel rods, the insert being adapted to be secured to the straps for extending in at least one cell. The insert extends along an axis intended to be parallel to that of a cell and comprises two axially spaced end portions for connecting the insert to the straps and at least one elongated blade-like spring extending axially between the end portions for supporting a fuel rod. The spring has a non-rectilinear cross-section in each plane perpendicular to the insert axis.