We analyze supermembrane instantons (fully wrapped supermembranes) by computing the partition function of the three-dimensional supersymmetricU(N)matrix model under periodic boundary conditions. By mapping the model to a cohomological field theory and considering a mass-deformation of the model, we show that the partition function exactly leads to the U-duality invariant measure factor entering supermembrane instanton sums. On the other hand, a computation based on the quasi-classical assumption gives the non U-duality invariant result of the zero-mode analysis by Pioline &etal; [2]. This is suggestive of the importance of supermembrane self-interactions and shows a crucial difference from the matrix string case. ©2002 American Institute of Physics.