Context : Aims : We searched for very high energy ( VHE ) \gamma -ray emission from the supernova remnant Cassiopeia A Methods : The shell-type supernova remnant Cassiopeia A was observed with the 17 meter MAGIC telescope between July 2006 and January 2007 for a total time of 47 hours . Results : The source was detected above an energy of 250 GeV with a significance of 5.2 \sigma and a photon flux above 1 TeV of ( 7.3 \pm 0.7 _ { stat } \pm 2.2 _ { sys } ) \times 10 ^ { -13 } cm ^ { -2 } s ^ { -1 } . The photon spectrum is compatible with a power law dN/dE \propto E ^ { - \Gamma } with a photon index \Gamma =2.3 \pm 0.2 _ { stat } \pm 0.2 _ { sys } . The source is point-like within the angular resolution of the telescope . Conclusions :