Recent developments in the Swampland program motivate the existence of a mesoscopic Dark Dimension of size $0.1-10\mu$m with bulk Majorana fermions in the $1-10$~keV range. Motivated by this, we derive terrestrial constraints on Majorana bulk neutrinos as a function of the compactification radius and confront them with the preferred region of the Dark Dimension scenario. After determining the mass spectrum and mixing structure, we confront the model with Daya Bay neutrino-oscillation data, the KATRIN beta-decay bound, and the KamLAND-Zen neutrinoless double beta decay limit. For the latter, we obtain a closed-form expression for the effective Majorana mass and show that the Kaluza–Klein tower efficiently screens neutrinoless double beta decay when the nuclear momentum exceeds the compactification and Majorana scales. In the Dark-Dimension window, beta decay provides the strongest constraint, and, together with the measured neutrino mass splittings, it restricts the largest brane Yukawa coupling to $𝒪({10}^{-3})$, with a weak dependence on the compactification scale. Oscillation and neutrinoless double beta decay searches remain complementary, probing smaller and larger Majorana masses, respectively. For completeness, outside the Dark-Dimension assumptions, we investigate how hierarchical Majorana masses can weaken the constraints.