Polymers responsive to external stimuli (e.g., electric field, chemical vapor, light) are of great interest for smart materials such as sensors and soft robotics. A vapor-driven multilayer polymer actuator, capable of fast and large-scale actuation, is described here. This Janus-like actuator is prepared with two polyelectrolyte multilayer systems (polyethylenimine (PEI)/poly(acrylic acid) (PAA) and polyurethane (PU)/poly(acrylic acid) (PAA)) using layer-by-layer assembly (LbL). The differing hydrophilicity of these two nanocoatings results in different swelling behavior in water and organic solvents, which leads to vapor-responsive mechanical motion. The bending/curling degree of this polymeric actuator can be precisely controlled by changing the thickness ratio of the two layers. A vapor sensor was constructed to demonstrate the environmental detection ability of this unique actuator.