  We show that in supersymmetric unified theories such as SO(10),
implementation of the see-saw mechanism for neutrino masses introduces a new
set of color triplet fields and thereby a new source of d=5 proton decay
operators. For neutrino masses in a plausible range, these operators are found
to have the right strength to yield observable, but not yet excluded, proton
decay rates. The flavor structure of the new operators is distinctive. Proton
decay modes into a charged lepton, such as l^+ \pi^0, l^+ K^0 and l^+ \eta
where l = e or \mu, can become prominent, even for low or moderate values of
tan(\beta) < 10, along with the \nu K^+ and \nu \pi^+ modes. A distinctive
feature is the charged lepton modes involving an e^+ and/or a \mu^+ with the
ratio \Gamma(l^+ K^0):\Gamma(l^+ \pi^0) = 2:1.
