We study quantum transport of an interacting Bose-Einstein condensate in a two-dimensional disorder potential. In the limit of a vanishing atom-atom interaction, a sharp cone in the angle-resolved density of the scattered matter wave is observed, arising from constructive interference between amplitudes propagating along reversed scattering paths. Weak interaction transforms this coherent backscattering peak into a pronounced dip, indicating destructive instead of constructive interference. We reproduce this result, obtained from the numerical integration of the Gross-Pitaevskii equation, by a diagrammatic theory of weak localization in the presence of nonlinearity. DOI: 10.1103/PhysRevLett.101.020603 PACS numbers: 05.60.Gg, 03.75.Kk, 67.85.ÿd, 72.15.Rn The past years have witnessed an increasing number of theoretical and experimental research activities on the behavior of ultracold atoms in magnetic or optical disorder potentials [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15]. A central aim in this context is the realization and unambiguous identification of strong Anderson localization with Bose-Einstein condensates, which was attempted by several experimental groups [1][2][3] with recent success [4,5], and theoretically studied both from the perspective of the expansion process of the condensate [6,7] as well as from the scattering perspective [8,9]. Complementary studies were focused on localization properties of Bogoliubov quasiparticles [10,11], on dipole oscillations in the presence of disorder [12,13], as well as on the realization of Bose glass phases [14,15].The above-mentioned topics (apart from Ref.[7]) mainly refer to processes that are essentially one dimensional (1D) by nature. Qualitatively new phenomena, however, do arise in two or three spatial dimensions, due to the scenario of weak localization. The latter manifests in a slight reduction of the transmission probability of an incident wave through a disordered region as compared to the classically expected value, due to constructive interference between backscattered paths and their time-reversed counterparts. This interference phenomenon particularly leads to a cone-shaped enhancement of the backscattering current in the direction reverse to the incident beam, which was indeed observed [16] and theoretically analyzed [17] in light scattering processes from disordered media. Related weak localization effects also arise in electronic mesoscopic physics, leading to characteristic peaks in the magnetoresistance [18,19].In this Letter, we investigate the phenomenon of coherent backscattering with atomic Bose-Einstein condensates that propagate in presence of two-dimensional (2D) disorder potentials. An essential ingredient that comes into play here is the interaction between the atoms of the condensate. On the mean-field level, this is accounted for by the nonlinear term in the Gross-Pitaevskii equation describing the time evolution of the condensate wave function. Indeed, nonlinearities do also appear in scattering processes of light, e....