The vapor-phase polymerization of pyrrole in the presence of polystyrene (PS) particles adsorbed at the air-water interface successfully leads to the formation of PS/polypyrrole (PPy) Janus particles where only the surface in contact with the air phase is regioselectively covered with the PPy nanolayer. The coverage area of the PPy nanolayer on the Janus particles, and therefore the contact angle of Janus particles at the air-liquid interface, decreases with a decrease in surface tension of liquid by the addition of isopropanol. The contact angle of particles at the interface decreases after the polymerization, which should be because the pyrrole monomer dissolved in the water phase from the gas phase promotes the wetting of the liquid to the PS particles by decreasing the surface tension of the liquid. Controlling the PS particle size realizes the formation of PS/PPy Janus particles with sizes ranging between 5 and 1000 μm. Furthermore, the Janus particles are demonstrated to be oriented at the air-water interface with the hydrophilic PS side toward the water phase and the hydrophobic PPy side toward the air phase, realizing stabilization of an armored bubble in the water medium.