It was demonstrated that a thin film of chemically exfoliated graphene flakes, blended with poly(methyl methacrylate), could be used as a chemiresistor sensor for the sensitive and selective determination of formaldehyde vapor. Formaldehyde adsorption on the sensing film led to an increase in the electrical resistance of the sensing film. It was demonstrated that direct interaction of formaldehyde with graphene flakes was responsible for the observed response. Graphene/polymer ratio was found to be important parameter in defining the dominant sensing mechanism and sensor performance. A reasonable relationship was found between formaldehyde concentration and electrical resistance response of the sensor. The amounts of graphene and polymer in the sensing composite were optimized by using three-level full factorial design in order to acquire higher sensitivity and selectivity. Final sensor response for formaldehyde concentration was linear between 0.05 and 5.0 ppm. The detection limit of this sensing device was calculated equal to 10 ppb.