Developing a new strategy to improve the self‐assembly efficiency of functional assemblies in a confined space and construct hybrid functional materials is a significant and fascinating endeavor. Herein, we present a highly efficient strategy for achieving the supramolecular self‐assembly of well‐defined metallacages in microdroplets through continuous‐flow microfluidic devices. The high efficiency and versatility of this approach are demonstrated by the generation of five representative metallacages in different solvents containing water, DMF, acetonitrile, and methanol in a few minutes with nearly quantitative yields, in contrast to the yields obtained with the hour‐scale reaction time in a batch reactor. A ring‐opening catalytic reaction of the metallacages was selected as a model reaction for exploring supramolecular catalysis in microdroplets, whereby the catalytic yield was enhanced by 2.22‐fold compared to that of the same reaction in the batch reactor. This work illustrates a new promising approach for the self‐assembly of supramolecular systems. Supramolecular self‐assembly of well‐defined metallacages in microdroplets was achieved through a continuous‐flow microfluidic approach. The high efficiency and diversity of this strategy were illustrated by the generation of five representative metallacages with nearly quantitative yields in a few minutes. This approach is promising for the self‐assembly of supramolecular systems in a confined space.