Context. A giant planet was recently discovered around the young star beta Pictoris. This planet is the closest to its parent star ever imaged. With an estimated mass of about 9 MJup and separation of 8-15 AU, it explains most of the peculiarities of beta Pictoris and its disk. Aims: Previous detections were made in the L' band (3.8 mum) and at 4.05 mum. We recorded new Ks-band data (2.18 mum) in order to measure its color and get an additional estimate of its mass and effective temperature Methods: Angular differential Ks-band images of beta Pictoris were recorded with NaCo in March and April 2010. Results: The companion is detected at Ks. This independently confirms the physical nature of beta Pictoris b inferred from the L' and NB_4.05 bands. The increase of the projected separation between October-December 2009 and April 2010 observations is consistent within error bars with the expected orbital motion. Using the absolute Ks photometry, "hot start" evolutionary models predict a mass of 7-11 MJup in agreement with previous estimates. Moreover, this mass is compatible with Teff = 1700 ± 300 K derived from the comparison of the Ks - L' color with those generated using synthetic spectra. Based on observations collected at the European Southern Observatory, Chile, ESO. Program ID: 284.C-5057.