By coherently combining the light of several telescopes on long baselines, interferometry makes it possible to reach unprecedented angular resolutions in the infrared (down to 3 milliarcseconds at 3 microns). The VLTI, with its four 8-m telescopes over 100-m baselines, has allowed to separate the light of several exoplanets from their overwhelmingly bright host stars, and to obtain their K-band spectra at medium spectral resolution with the GRAVITY instrument. This led to precise astrometric fitting of their orbits and spectroscopic determination of their C/O ratios, which are essential to understand the structure of their atmospheres and how they formed in their protoplanetary disk. To improve this atmospheric retrieval, extending the observations to the mid infrared is a great asset. This recently became possible with the MATISSE instrument and the GRA4MAT narrow-angle subsystem, which allows one to: a) observe a faint planet with MATISSE while tracking the interferometric fringes on the bright star with GRAVITY, providing longer exposure times and a larger sensitivity; and b) to lower the stellar contribution in the correlated flux by offsetting the star from the instrument spatial filter. We are presenting in this poster the first exoplanet observations with MATISSE and GRA4MAT on the well-known Beta Pictoris b, a young giant planet surrounded by a debris disk. We will show how we retrieve the planet’s L-band spectrum and which new information it provides on its atmospheric composition and origin.