Intrigued by marine mussel's iron-fortified adhesive system, a novel approach to polymer–inorganic hybrid membrane was developed using polymeric material as the membrane bulk matrix and Fe³⁺–dopamine organometallic nanoaggregates as the versatile filler. The structures of the hybrid membrane were precisely tuned by varying the composition of the organometallic nanoaggregates, resulting in the well-controlled size-dependent transport of gas molecules. This finding may have great implications for design of new generation hybrid membranes, particularly for applications as highly permeable and selective membranes for CO₂ capture.