Driven by gravity, massive structures like galaxies and clusters of galaxies are believed to grow continuously through hierarchical merging and accretion of smaller systems. Observational evidence of accretion events is provided by the coherent stellar streams crossing the outer haloes of massive galaxies, such as the Milky Way 1 or Andromeda 2 . At similar mass-scales, around 10 11 solar masses in stars, further observational evidence of merging activity is also ample 3,4,5 . Mergers of lower-mass galaxies are expected within the hierarchical process of galaxy formation 6 , but have hitherto not been seen for galaxies with less than about 10 9 solar masses in stars 7,8 . Here, we report the kinematic detection of a stellar stream in one of the satellite galaxies of Andromeda, the dwarf spheroidal galaxy Andromeda II, which has a mass of only 10 7 solar masses in stars 9 . The properties of the stream show that we are observing the remnant of a merger between two dwarf galaxies. This had a dramatic influence on the dynamics of the remnant, which is now rotating around its projected major axis 10 . The stellar stream in Andromeda II illustrates the scale-free character of the formation of galaxies, down to the lowest galactic mass scales.Andromeda II (And II) is, in size, the second largest dwarf spheroidal galaxy known in the Local Group, with a half light radius 11 of about 1.2 kpc (second only to And XIX 12 ). With a luminosity 9 of L V = 9.4 × 10 6 L ⊙ , it currently sits 13 at a distance of 185 kpc from its host, and at an heliocentric distance of about 650 kpc. Among the satellites of M31, And II is one of the few for which a spectroscopic dataset of hundreds of stars is currently available. These observations, made using the Deep Imaging Multi-Object spectrograph (DEIMOS) on the Keck II telescope, revealed a strong and puzzling stellar rotation, so far unique among the dwarf spheroidal galaxies of the Local Group 10 . The results presented here are based on a re-analysis of the latter spectroscopic observations, kindly provided by N. Ho and M. Geha.We assign reliable probabilities of membership to all spectroscopic targets, comprising more than 700 candidate red giant branch stars, by allowing for the presence of both foreground contaminants from the Milky Way and interlopers from the halo population of M31. Each of the three coexisting components (members, Milky Way halo contaminants, M31 halo contaminants) -2 -are described by a distinct spatial and kinematical distribution, the parameters of which are measured through a maximum likelihood technique (see Methods). A Bayesian approach then allows us to estimate a probability of membership for each available star: we count 632 high probability members (p>0.85; see Extended Data Fig. 1).We then study the kinematical properties of And II also through a maximum likelihood method that fully takes into account the observational uncertainties on the line-of-sight velocity of each available giant star in the spectroscopic sample. We find that, within measurement ...