Herein we discuss band gap modification of MIL-125, a TiO2/1,4-benzenedicarboxylate (bdc) metal–organic framework (MOF). Through a combination of synthesis and computation, we elucidated the electronic structure of MIL-125 with aminated linkers. The band gap decrease observed when the monoaminated bdc-NH2 linker was used arises from donation of the N 2p electrons to the aromatic linking unit, resulting in a red-shifted band above the valence-band edge of MIL-125. We further explored in silico MIL-125 with the diaminated linker bdc-(NH2)2 and other functional groups (−OH, −CH3, −Cl) as alternative substitutions to control the optical response. The bdc-(NH2)2 linking unit was predicted to lower the band gap of MIL-125 to 1.28 eV, and this was confirmed through the targeted synthesis of the bdc-(NH2)2-based MIL-125. This study illustrates the possibility of tuning the optical response of MOFs through rational functionalization of the linking unit, and the strength of combined synthetic/computational approaches for targeting functionalized hybrid materials.