The invention relates to a derivative of a FGF21 protein having a cysteine residue at a position corresponding to position 167, 169, 170, 171, 172, 173, 174, 175 and in particular position 180 or position 181 of mature human FGF21 and derivatives thereof having a side chain attached to this cysteine. The FGF21 derivatives of the invention display high potency towards the FGF receptors. The invention also relates to pharmaceutical compositions comprising such FGF21 derivatives and pharmaceutically acceptable excipients, as well as the medical use of the FGF21 derivatives.