  We demonstrate viability of the one-loop neutrino mass mechanism within the
framework of grand unification when the loop particles comprise scalar
leptoquarks (LQs) and quarks of the matching electric charge. This mechanism
can be implemented in both supersymmetric and non-supersymmetric models and
requires the presence of at least one LQ pair. The appropriate pairs for the
neutrino mass generation via the up-type and down-type quark loops are
$S_3$-$R_2$ and $S_{1,\,3}$-$\tilde{R}_2$, respectively. We consider two
phenomenologically distinct regimes for the LQ masses in our analysis. First
regime calls for very heavy LQs in the loop. It can be naturally realised with
the $S_{1,\,3}$-$\tilde{R}_2$ scenarios when the LQ masses are roughly between
$10^{12}$ GeV and $5 \times 10^{13}$ GeV. These lower and upper bounds
originate from experimental limits on partial proton decay lifetimes and
perturbativity constraints, respectively. Second regime corresponds to the
collider accessible LQs in the neutrino mass loop. That option is viable for
the $S_3$-$\tilde{R}_2$ scenario in the models of unification that we discuss.
If one furthermore assumes the presence of the type II see-saw mechanism there
is an additional contribution from the $S_3$-$R_2$ scenario that needs to be
taken into account beside the type II see-saw contribution itself. We provide a
complete list of renormalizable operators that yield necessary mixing of all
aforementioned LQ pairs using the language of $SU(5)$. We furthermore discuss
several possible embeddings of this mechanism in $SU(5)$ and $SO(10)$ gauge
groups.
