Electrochemically synthesized polypyrrole (PPy) films undergo rapid and intensive bending in response to sorption of water vapor, which is applied to a new type of motor capable of transducing chemical free energy change of sorption directly into mechanical work. Furthermore, the PPy film contracts in air under an electric field, which is associated with desorption of water vapor due to Joule heating. This paper features an overview of our comprehensive study on the humidity-sensitive PPy films and applications to electro-active polymer actuators. Upon application of 3 V, the film generates contractile stress of 9.8 MPa which is 4 orders of magnitude larger than the gram-force of its own weight (7.6 mgf) and is nearly 30 times that of skeletal muscle in animals (0.35 MPa). The work capacity increases with the applied voltage and reaches 48 kJ m−3 at 3 V, while the degree of contraction is about 1%. The ‘origami’ actuator fabricated by folding the PPy film exhibits a significant expansion as large as 147% at 2 V, which lies in the electrically induced changes in the elastic modulus of the humidity-sensitive PPy film. Utilizing the origami technique, a biomorphic origami robot is fabricated, which can move with a caterpillar-like motion by repeated expansion and contraction at a velocity of 2 cm min−1.