We study a 3D fluid-rigid body interaction problem. 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 describing conservation of linear and angular momentum. Our aim is to prove that any weak solution satisfying certain regularity conditions is smooth. This is a generalization of the classical result for the 3D incompressible Navier-Stokes equations, which says that a weak solution that additionally satisfy Prodi -Serrin L r − L s condition is smooth. We show that in the case of fluid -rigid body the Prodi -Serrin conditions imply W 2,p and W 1,p regularity for the fluid velocity and fluid pressure, respectively. Moreover, we show that solutions are C ∞ if additionally we assume that the rigid body acceleration is bounded almost anywhere in time variable. * 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), 22-01591S. Moreover, Š. N. has been supported by Praemium Academiae of Š. Nečasová. CAS is supported by RVO:67985840.‡ The research of A.R. leading to these results has been supported by Croatian Science Foundation under the project IP-2019-04-1140. Moreover, The research of A.R. leading to these results has received funding from the Czech Sciences Foundation (GA ČR) 22-01591S, and by Praemium Academiae of Š. Nečasová.