Formaldehyde is one of the toxic volatile organic compounds and is known to be a carcinogen and a major cause of sick building syndrome. In this study, polystyrene (PS)/polyaniline (PANI) core-shell type microparticles with narrow size distributions were prepared by applying the Swelling-Diffusion-Interfacial Polymerization Method (SDIPM). The PANI shell was uniformly coated on the PS core and the PANI loading efficiency estimated by gravimetric reaction yield analysis was 98.69%. A formaldehyde sensor device was fabricated by a solution casting of the PS/PANI core-shell type microparticles on the Interdigitated Electrodes (IDEs). The PS/PANI coated IDE sensor showed an excellent sensitivity for formaldehyde in range of 1–50 ppm at room temperature as follows: −9.9% of response for 1 ppm and −37.6% of response for 50 ppm. In addition, it showed a clear reversibility indicating that the sensor could be reused at least 3 times for the detection of formaldehyde. Display omitted •Polystyrene/polyaniline core-shell microparticles showing narrow size distribution and uniform shell thickness.•Formaldehyde sensor device via a solution casting of the PS/PANI core-shell microparticles on the interdigitated electrodes.•PS/PANI coated IDE formaldehyde sensor showing high sensitivity, practical controllability, and clear reversibility.•PS/PANI coated IDE formaldehyde sensor applicable to the application area requiring a bendable form.