Nanoscale metal-organic frameworks (NMOFs) have attracted increased attention in recent years for miniaturized and/or biological applications. However, the synthesis of ultrasmall NMOFs with good stability is a great challenge. In this study, sub-5-nm nano-HKUST-1 was prepared for the first time via a mild metal- organic gel route without surfactants or capping agents. Controlling the gelation process via anion–ligand self-assembly is the key to the formation of NMOFs. The Tyndall effect, zeta potential, and liquid adsorption indicated strong stability of the obtained nano-HKUST-1, even in water. Adsorption experiments were performed using different dyes (crystal violet and methylene blue) to demonstrate the size-dependent adsorption thermodynamics and kinetics of this famous MOF. The results of this study provide new insights regarding the synthesis of NMOFs and their efficient applications.