  Motivated by the disagreement between experimental and lattice QCD results on
the D_s decay constant we systematically reinvestigate role of leptoquarks in
charm meson decays. We consider scalar leptoquarks that transform as a weak
interaction triplet, doublet, or singlet in a model independent approach, and
also argue that in a particular SU(5) GUT model these leptoquark states,
contained in the 45-dimensional Higgs representation, could be safe against
proton decay bounds. Using the current experimental measurements in tau, kaon
and charm sectors, we find that scalar leptoquarks cannot naturally explain the
D_s --> mu nu and D_s --> tau nu decay widths simultaneously. While any
contributions of the triplet leptoquarks are already excluded, the singlets
could only contribute significantly to the D_s --> tau nu width. Finally, a
moderate improvement of the experimental upper bound on the D^0 --> mu^+\mu^-
decay width could exclude the doublet contribution to the D_s --> mu nu, while
present experimental data limits its mass to be below 1.4 TeV. Possible new
signatures at present and near future experiments are also briefly discussed.
