We study a 3D nonlinear moving boundary fluid-structure interaction problem describing the interaction of the fluid flow with a rigid body. The fluid flow is governed by 3D incompressible Navier-Stokes equations, while the motion of the rigid body is described by a system of ordinary differential equations called Euler equations for the rigid body. The equations are fully coupled via dynamical and kinematic coupling conditions. We consider two different kinds of kinematic coupling conditions: no-slip and slip. In both cases we prove a generalization of the well-known weak-strong uniqueness result for the Navier-Stokes equations to the fluid-rigid body system. More precisely, we prove that weak solutions that additionally satisfy Prodi-Serrin L r − L s condition are unique in the class of Leray-Hopf weak solutions. * The research of B.M. leading to these results has been supported by Croatian Science Foundation under the project IP-2018-01-3706† The research of Š.N. leading to these results has received funding from the Czech Sciences Foundation (GA ČR), 19-04243S and RVO 67985840.‡ The research of A.R. leading to these results has been supported by Croatian Science Foundation under the project IP-2018-01-3706, and by the Czech Sciences Foundation (GA ČR), 19-04243S and RVO 67985840.