Introduction of functionalized mesopores into microporous metal-organic frameworks (MOFs) can endow them with suitable properties for applications in gas storage, separation, catalysis, and drug delivery. However, common methods for functionalization (including pre- and post-synthetic modifications) of the internal surface of a MOF reduce the pore size of the MOF because the additional functional groups fill up the pores. We present a metal-ligand-fragment coassembly strategy for the introduction of (meso)pores functionalized with various substituent groups on the ligand fragments. Astonishingly, this new functionalization strategy increases the pore volume of a MOF instead of reducing it. Since the ligand fragments are often readily available or easily prepared, the new procedure for synthesis of the modified MOFs becomes much easier and more applicable than existing approaches. Remarkably, mesopores can be generated conveniently and controllably by the coassembly of a ligand and its fragment containing the desired functional groups. The fragment/ligand ratio has been optimized to preserve the parent structure and to promote maximum mesopore introduction, which has led to a systematic evaluation of the effectiveness of a series of functional groups for the adsorption of guest molecules.