Spectrum of β Pic b with MATISSE (black, this work) compared to other instruments: NACO (yellow, new data), GPI (blue, Chilcote et al. 2017), GRAVITY (green, new data) and MIRI (red, Worthen et al. 2024). Only the slope of the NACO spectrum can be compared to MATISSE as the absolute level was scaled manually. MATISSE data with a S/N < 5, located in strong telluric bands and not used in this work, are shown in grey. The absorption curves of H₂O, CO, and CH₄, calculated from the best-fitting Exo-REM model presented in Sect. 5, are plotted at the top to visualize their impact on the spectrum. H₂O and CO leaves a visible pattern, but not CH₄, likely because it is dominated by H₂O absorption or buried in noise.