Increasing attention has been dedicated to the development of nanomaterials rendering green and sustainable processes, which avail in benign aqueous reaction media. Herein, we demonstrate the synthesis of a novel family of green nanomaterials, layered double hydroxide (LDH) nanoclusters, which are concentrated (98.7 g/L in aqueous solvent), stably-dispersed (transparent sol for >2 weeks), and catalytically-active colloids of nano LDHs (isotropic shape with the size of 7.8 nm by SAXS). LDH nanocluster is available as colloidal building blocks to give access to meso/macroporous LDH materials. Proof-of-concept applications revealed that the LDH nanocluster works as a solid basic catalyst and is separable from solvents of catalytic reactions, confirming the nature of nanocatalysts. The present work closely investigates the unique physical/chemical features of this colloid, the formation mechanism, and the capability as basic nanocatalysts in benign aqueous reaction systems.