In order to probe the inside-out formation of the most massive galaxies in the Universe, we have explored the radial (0.1 ≲ R ≲ 8 kpc) variation of the spectral energy distribution of M87 from UV to IR. For this purpose, we have combined high-resolution data in 16 different bands. Our analysis indicate that the age of the stellar population of M87 remains almost unchanged with radius. However, the metallicity ([Z/H]) profile presents three different zones: the innermost kpc shows a plateau with supersolar metallicity, followed by a decline in metallicity down to 5 kpc and another plateau afterwards. The size of the inner plateau is similar to the expected size (Re) of an object with the predicted mass of M87 at z = 2. The global [Z/H] gradient is -0.26 ± 0.10, similar to those found in other nearby massive ellipticals. The observed change in the stellar population of M87 is consistent with a rapid formation of the central part (R ≲ 5 kpc) of this galaxy followed by the accretion of the outer regions through the infall of more metal-poor material.