Whether the 3D incompressible Navier-Stokes equations will have a global smooth solution for all smooth,finite energy initial data is a Millennium Prize problem. One of the main difficulties of this problem is that the Navier-Stokes equations are actually a system of semilinear heat equations rather than a single equation. In this paper, we discover a remarkable hidden symmetry of the 3D incompressible Navier-Stokes equations. Under this symmetric reduction,the system reduces to a single scalar semilinear heat equation. The symmetry also holds for the 3D incompressible Euler equations.