Context. A giant planet was recently discovered around the young star β 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 β Pictoris  and its disk. Aims. Previous detections were made in the L′ band (3.8 μm) and at 4.05 μm. We recorded new Ks-band data (2.18 μm) in order to measure its color and get an additional estimate of its mass and effective temperature Methods. Angular differential Ks-band images of β Pictoris were recorded with NaCo in March and April 2010. Results. The companion is detected at Ks. This independently confirms the physical nature of β 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.