A comparison of the structure and sensitivity of humidity sensors prepared from graphene (G)-PEDOT: PSS (poly (3,4-ethylenedioxythiophene)) composite material on flexible and solid substrates is performed. Upon an increase in humidity, the G: PEDOT: PSS composite films ensure a response (a linear increase in resistance versus humidity) up to 220% without restrictions typical of sensors fabricated from PEDOT: PSS. It was found that the response of the examined sensors depends not only on the composition of the layer and on its thickness but, also, on the substrate used. The capability of flexible substrates to absorb the liquid component of the ink used to print the sensors markedly alters the structure of the film, making it more porous; as a result, the response to moisture increases. However, in the case of using paper, a hysteresis of resistance occurs during an increase or decrease of humidity; that hysteresis is associated with the capability of such substrates to absorb moisture and transfer it to the sensing layer of the sensor. A study of the properties of G: PEDOT: PSS films and test device structures under deformation showed that when the G: PEDOT: PSS films or structures are bent to a bending radius of 3 mm (1.5% strain), the properties of those films and structures remain unchanged. This result makes the composite humidity sensors based on G: PEDOT: PSS films promising devices for use in flexible and printed electronics.