Functionalized diamondoids show great potential as building blocks in view of their optoelectronic, catalytic, and therapeutic applications. In this work, we report the effect of functionalization on the electronic and absorption properties of the first two lower diamondoids, adamantane and diamantane, respectively. Weperformed quantum mechanical calculations within the framework of density functional theory (DFT) and timedependent (TD-DFT) theory for the mono-, di-, and tetra-substituted smaller diamondoids. Bandgap tuning is made through the push (electron-donating) and pull (electron-withdrawing) doping, external substitution at the C-H site, which reduces the band gap of diamondoids significantly. The stability of pristine and the functionalized smaller diamondoids is also studied by calculating the ionization potentials and affinity energies. Employing TD-DFT method at the level of PBE1PBE theory, the UV absorption spectra of the functionalized smaller diamondoids were calculated and discussed. While the lowest allowed and most intense transition energies are significantly reduced with the increasing number of pull-type functional groups, the push-type groups slightly increase those energies.